WASHINGTON — It's long been said that midterm elections are all about the current occupant of the White House - a referendum on the incumbent and his party. But do the old rules and assumptions apply as they once did? Because of Donald Trump, they may not this year.

American politics can be divided crudely into two eras: BDT and SDT, or Before Donald Trump and Since Donald Trump. What was true before he came on the scene isn't necessarily true now. Trump broke rules and assumptions on his way to winning the White House, broke more in office and is still breaking them. And that could haunt the Republicans in November.

Trump has been the energizing force in politics since he announced for president in 2015, mobilizing voters behind his candidacy and once in office triggering an even bigger backlash against him. This November's election will still be a reckoning for President Joe Biden and the Democrats, given inflationary pressures and disapproval with the incumbent's job performance. But Republicans cannot escape the reality that Trump and his Make America Great Again, or MAGA, movement are also part of the reckoning that will take place.

Since Trump came on the scene, elections are louder and angrier and, notably, they have drawn millions more Americans to the polls. In 2016, about 137 million Americans voted in the presidential election, compared with around 130 million in 2008 and 2012. In 2020 turnout spiked to 158 million. Biden got 15.4 million more votes than Hillary Clinton got in 2016 and Trump drew 11.2 million more in 2020 than he got in his first campaign. The Democrats' popular vote margin rose from nearly 3 million in 2016 to 7 million in 2020.

The presidential race was not an isolated example of the Trump factor. Just as startling was what happened in 2018. For decades, midterm election turnout, which is always lower than in presidential years, fluctuated within a relatively narrow range: From one midterm election to another, turnout rarely went up or down by more than a few percentage points. Then came 2018, when overall turnout was the biggest in roughly a century, registering an 11-point increase over 2014, according to census data.

This too was the Trump factor - in this case a revolt against him led by women voters that reshaped the contours of an election. By one calculation from the Democratic firm Catalist, Democrats gained 23 million more votes than in 2014 and Republicans added about 11 million. Trump wasn't on the ballot, but he was the biggest motivating force.

Some people might say that what happened in 2018 was a reflection of long-standing trends in midterm elections, though perhaps on steroids - an unpopular incumbent whose party took a beating. True. For Republicans, that has fed the hope that this November will be the reverse of 2018, another thumping for the party of another president with low approval ratings.

Though no one can predict whether turnout this fall will even come close to what happened in 2018, there are signs all around that this will be another SDT (Since Donald Trump) election and not necessarily one that conforms to what was the norm previously.

Republicans began the year with lofty expectations, built on traditional assumptions: Biden's approval ratings were deeply underwater and the inflation rate was rising to its highest levels in 40 years, even as the economy continued to add jobs at a healthy clip. Republican leaders talked expansively about playing offense in 70 or more congressional districts.

Those calculations were seen by independent analysts as overly rosy if only because they meant that the GOP would be going after seats in districts Biden had won by comfortable margins in 2020. But it wasn't foolhardy to think that under traditional rules of engagement in midterms, Republicans had clear advantages. Even many Democrats lamented how bad the climate seemed for their party.

Earlier in the year, White House officials concluded that the "MAGA" label was toxic to many voters and that, if it was broadly and effectively applied to the Republican Party, it could change the midterm election from a pure referendum on Biden to a choice between two philosophies and, presumably, two leaders, both unpopular.

On Thursday, Biden delivered a slashing speech in the Maryland suburbs that highlighted the White House's plan to employ this strategy over the next two-plus months. He described the Trump-led Republican Party as having taken a turn toward "semi-fascism" and said, "The MAGA Republicans don't just threaten our personal rights and economic security. They're a threat to our very democracy."

That message is one half of what White House officials see as the most effective way to wage the midterm campaign. The other will be to focus on Democrats' recent legislative successes and, if the numbers hold up, to point to a decline in gasoline prices to offset voters' worries about this year's high inflation.

Biden alone cannot change the midterm from a referendum on his presidency to a choice election. But he has an unexpected partner in this effort: Trump and the Republicans themselves. Trump remains in the forefront of this election year, continuing his baseless claims about a stolen election, caught up in twin Justice Department investigations over his retention of classified documents and the Jan. 6, 2021, attack on the Capitol, and demonstrating that the Republican Party is now very much the Trump Party through the power of his endorsements to prop up questionable candidates.

Republican primary voters, guided by Trump's endorsements, have in a variety of states nominated election deniers as candidates, who, if elected in the fall, will have control over the 2024 elections. Those nominations have added ammunition to Biden's and the Democrats' charge that the Republicans have become a MAGA-dominated political party.

Trump has been under the spotlight, as well, through the public hearings by the House committee investigating the Jan. 6, 2021, attack on the Capitol. The hearings have shown the efforts to which Trump and those close to him went to overturn the results of the 2020 election - and the degree to which the 2024 election could be put at risk if Trump acolytes control the administration of elections.

Meanwhile, the ongoing investigation by the Justice Department into Trump's retention of highly classified documents at his Mar-a-Lago estate in Florida has kept the former president in the forefront of the news. The search of the premises has touched off what is now a weeks-long story that is likely to continue for weeks more. Trump not only broke rules of politics, he may have broken the law.

The other factor that has changed the landscape - the Supreme Court's ruling that overturned Roe v. Wade - also has Trump's fingerprints on it. The three justices nominated he nominated - Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett - provided the clear margin for the ruling written by Justice Samuel Alito.

The decision in Dobbs v. Jackson Women's Health Organization has produced a surge in registration among female voters in a number of states and has become a motivating force for many women and men this fall, especially independent voters. The overwhelming vote earlier this month to maintain abortion rights in the Kansas constitution is the clearest sign of the power of the issue.

The Kansas vote was unique and doesn't translate directly to candidate vs. candidate contests. But the recent Democratic victory in a House special election in New York state, where abortion was a central issue, provided another indication of the power of the issue to redraw assumptions about November and has spooked Republicans.

Biden's approval ratings have improved in recent weeks but still threaten to be a drag on Democratic candidates. The Post recently reported that most Democratic candidates would prefer to campaign on their own, rather than invite Biden into their states. The president's campaign rally in deep blue Montgomery County on Thursday may be an exception to that pattern.

But weak approval ratings might not be as definitive an indicator of these midterm elections as those in the past. Democratic strategists have seen approval ratings of some candidates rise,even as Biden's were falling, suggesting that the candidates' fates may be somewhat decoupled from the president's ratings.

This is still a tough year for Democrats. But the polarizing effect of an ever-present and controversial former president means this midterm election may not conform to the norms of the past.