Editor's note: This is the first installation of "Facing the Future: Military matters facing the next president," a five-part series. Upcoming stories:

WASHINGTON — Presidential nominees Donald Trump and Hillary Clinton have pledged on the campaign trail to get tough about the nation’s defense. But by spring, the next president’s military plans could already be hamstrung by Capitol Hill politics.

Congress is weighing a controversial budget plan that siphons off war money to use for its other defense priorities next year — and leaves Trump or Clinton facing a fiscal cliff at the end of April when money would run out for troops fighting terrorists in Iraq, Syria and Afghanistan.

The cliff could be the winning candidate’s first taste of the politics that have snarled the defense budget in recent years, leading to Pentagon frustration, a rare presidential budget veto last year and threats of another veto this year.

Republican defense hawks, fiscal conservatives and Democrats are risking the fiscal cliff and a veto as they wage a high-stakes battle over a shrinking pool of defense dollars. The fight is likely to spill into next year — possibly intensified by the November election results — and could last throughout the next president’s first term.

It will shape what military options are available to Trump or Clinton, especially if the next administration wants major changes in the war against the Islamic State group and the forward deployment of forces in Europe and the Pacific region.

“In some areas, the president has a lot of discretion in how they manage the military and conduct foreign policy,” said Todd Harrison, director of defense budget analysis at the think tank Center for Strategic and International Studies. “When it comes to the budget, it depends much more on Congress and what Congress wants and what Congress wants to go along with.”

The cliff The clock is ticking on a new defense budget.

The current budget expires in October but the Republican-controlled House and Senate are sharply divided over which path to follow.

The $602 billion House plan pumps $18 billion into the military’s base budget. The extra money would mainly buy new hardware such as fighter jets, helicopters and ships — purchases the GOP contends will lead to a more prepared force.

It would also cover larger pay raises, more troops and increased operations and maintenance.

The plan would also hand the next president a fiscal cliff just months into his or her first term.

It pulls the $18 billion from an overseas war fund that supports operations against the Islamic State group in Iraq and thousands of troops that remain in Afghanistan.

The fund would run dry in April, leaving deployed forces in the lurch. A supplemental funding bill would be required from Congress.

Defense spending has been capped since 2013, so the war fund has become a last resort for Republican defense hawks, including Rep. Mac Thornberry, R-Texas, the chairman of the Armed Services Committee.

They say the military is underfunded, unprepared and in crisis, and are pushing to tap the war fund as a creative way to add more dollars.

“Unfortunately, our military is less ready than it should be to address the threats we face,” Thornberry wrote in an August column on the website Real Clear Defense. “It is too small to adequately meet the demands of a dangerous world, and men and women in uniform are being deployed into harm’s way without all the support they need and deserve.”

So far, the Senate is not on board with the plan.

It rejected the $18 billion boost and is instead backing a budget that sticks to the request made by the Pentagon and President Barack Obama. The president’s plan reduces base defense spending by 1.3 percent from the previous year.

The Senate’s approach might be partly to avoid political strife this fall.

Obama has threated to veto the defense budget — a promise he made good on last year — and Defense Secretary Ash Carter has repeatedly called Thornberry’s funding approach a “gamble” and “gimmick” that could endanger troops deployed to the Middle East and Afghanistan.

The caps War hawks such as Thornberry say they see a troubling military decline but have few options to counter it.

Defense spending reached a historic high during the wars in Iraq and Afghanistan but has fallen precipitously since then, according to an analysis by the Center for Strategic and Budgetary Assessments, a Washington think tank.

The decline has happened faster than the drop-off after the previous spending high, the Cold War military buildup in the 1980s under President Ronald Reagan’s administration, the center found.

Meanwhile, the services have gotten little rest after waging large-scale ground wars during the past decade. U.S. carriers have tried to keep a near permanent presence in the Middle East — and elsewhere — while a new war against the Islamic State group has broken out in Iraq and Syria, 8,400 troops remain in Afghanistan, the Army is moving armor back into Europe to counter Russian aggression and the Navy in the Pacific is facing Chinese maritime expansion.

“It’s the busiest Air Force that I have certainly ever seen in my 35 years of working on defense matters,” Secretary Deborah James said in August while warning the service will be short 1,000 fighter pilots during the next few years.

As military spending ramped down, Congress made a fateful decision in 2011.

It passed a decades’ worth of federal budget caps. They were designed to placate fiscal conservatives concerned U.S. spending is out of control and aimed to slash defense spending by a whopping $1 trillion by 2022.

Lawmakers on both sides of the aisle have widely decried the caps and since made a series of deals to avoid severe cuts.

Few of them have any hope of a long-term solution. A bloc of Tea Party conservatives in the House called the Freedom Caucus has held Congress’ feet to the fire by refusing to repeal the 2011 Budget Control Act, which spawned the spending limits.

A new deal? A new short-term spending deal from Congress could give the next president some breathing room.

Sen. John McCain, R-Ariz., is leading a group of senators negotiating the annual defense budget with Thornberry and House lawmakers.

McCain, chairman of the Armed Services Committee, tried to get the Senate to back the $18 billion defense budget hike in June with a dire floor speech about troops being sent into battle unprepared.

“Shamefully, our military is being forced to confront growing threats with shrinking resources,” McCain said.

It was the same message pressed successfully by Thornberry, but the Senate voted down the proposal.

What the final agreement between McCain and Thornberry might look like — or whether it supports a boost to defense spending — is now uncertain.

They are expected to unveil a bill as early as September and it will be subject to approval by Congress.

Justin Johnson, a senior policy analyst at the conservative Heritage Foundation, said lawmakers could find agreement on a defense hike somewhere below the total sought by Thornberry.

“If I were to place a bet on the outcomes this fall, I would guess there would be some sort … of number between that [$18 billion] and zero,” Johnson said.

But a deal between the Republican chairmen might fall flat with Democrats, who have moved to derail the budget process as the bitter politics on Capitol Hill have continued to sour.

Congress struck a deal in late 2015 that set defense spending levels and was supposed to head off gridlock for two years. But the deal crumbled within months.

In July, Senate Democrats blocked a defense appropriations bill, which forms half of the annual budget legislation and doles out money for the military. They are demanding any increases in defense are matched with dollar-for-dollar increases in domestic spending.

The move is a signal Democrats are abandoning individual budget bills drafted by the Republic majority and want an overarching omnibus bill, where they might have more leverage to negotiate increases.

Senate Minority Leader Harry Reid, D-Nev., provided a glimpse of the rancorous debate during a July floor speech. He traded barbs with Majority Leader Mitch McConnell, saying the Kentucky Republican was lying about the appropriations bill during the July maneuvering and complained about “right-wing crazies” in the House.

“I assume that my Republican friend feels that if you say just the opposite of what is valid and true some people will believe it,” Reid said.

The election The complex battle over defense funding could potentially stretch past the November election and into next year, leaving key funding questions unanswered as the next commander-in-chief assumes office.

An election shakeup on Capitol Hill is a wild card for the defense budget, though some experts believe it is likely to further snarl politics over spending.

Katherine Blakeley, a research fellow at the independent Center for Strategic and Budgetary Assessments, said Congress might abandon efforts to pass an annual budget this year and instead opt for a stopgap measure that maintains current spending levels for some part of 2017 until some other solution can be found.

“I think it is highly unlikely you will see any resolution to this budget stalemate,” Blakeley said.

As of August, it appeared the Democrats could win a majority of seats in November and take control of the Senate from Republicans, said Larry Sabato, a professor and director of the Center for Politics at the University of Virginia.

A Clinton win in the presidential race — polling in mid-September showed her ahead but in a tightening race — could give the party more leverage for their priorities in the Senate, Sabato said. That could mean more pressure on the GOP to allow more domestic spending as a prerequisite to any defense increases.

The election could bolster an equal and opposite political force in the House. Republicans could lose seats in that chamber but are likely to retain control, Sabato said.

Losses will most likely cull moderate House Republicans and leave an even more fiscally conservative bloc that shuns efforts to lift spending caps and fights the Democrats to gridlock, he said.

“It is possible that the Freedom Caucus will force [House Speaker] Paul Ryan to say ‘no’ to everything,” Sabato said.

So, voters will hand Trump or Clinton the presidency and an opportunity to steer defense policy. But their administration might find it increasingly difficult to get an annual military budget through Congress that matches its priorities.

Trump has talked about rebuilding the military but also wants to squeeze allies for bigger contributions and possibly scale back the American footprint abroad. Clinton has said she will pursue a similar military strategy as Obama but would intensify operations against the Islamic State group.

But the hard realities of Hill politics and declining military funding await, Johnson said.

“There is going to be a serious need to catch up [on military funding] for whoever is in the White House, and there’s the question of how to make that happen financially,” he said. “That first budget request is going to be addressing those issues head on.”

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