This week, Senate Democrats are going to try - and fail - to get rid of the filibuster for voting rights legislation. They need all 50 senators in the Democratic caucus on board, and Sen. Kyrsten Sinema, D-Ariz., and - to a slightly lesser degree - Sen. Joe Manchin, D-W.Va., are opposed to changing those rules.

But Manchin and Sinema did vote in December to make an exception to the filibuster to raise the debt ceiling, prompting some of their liberal critics to ask: Well, why not do it again for voting rights?

“If we can change the process on the debt ceiling,” said Sen. John Hickenlooper, D-Colo., recently, “then surely we can do the same to protect our democracy.”

“We just got around the filibuster to raise the debt limit,” Rep. James Clyburn, D-S.C., said on CNN this weekend.

But the two situations are not entirely analogous, and thus Manchin and Sinema are unlikely to be swayed. Here’s why.

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How the Senate lifted the filibuster for the debt ceiling

The background: In December, Congress was facing a potentially catastrophic default on U.S. debt if it didn’t pass a law raising the debt ceiling, and soon. A few months earlier, Senate Republicans had vowed to block raising the debt ceiling but had to back off at the last minute and provide 10 Republican votes to overcome a filibuster waged by other Republican senators.

The debt ceiling needed attention again in December, and some Republican senators said they’d filibuster that vote, too.

Rather than lose another showdown with Democrats over what to do, Senate Minority Leader Mitch McConnell, R-Ky., agreed to an elaborate deal with Senate Majority Leader Charles E. Schumer, D-N.Y., to raise the debt ceiling without Republican votes, but with Republicans voting to agree to get out of the way. So, they created a bill to bypass the filibuster for the debt ceiling.

How they did it: They created legislation specifically allowing the Senate to raise the debt ceiling without a filibuster. That legislation alone didn’t raise the debt ceiling; it simply set up how to do it with only Democratic votes. “The bill also establishes expedited Senate procedures for considering legislation to increase the debt limit. The procedures limit debate, waive points of order, and prohibit amendments. The procedures may only be used once and expire after January 16, 2022,” the legislation read. Eleven Republican senators and all 50 Democrats voted to advance this.

That included Manchin and Sinema, even though Manchin at least had previously said he wouldn’t support changing the filibuster for the debt ceiling.

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What’s different about waiving the filibuster for the debt ceiling vs. for voting rights

A few things.

A Senate Democratic aide, who spoke on the condition of anonymity to discuss internal deliberations, emphasized that the debate to lift the filibuster for the debt ceiling was a one-time, limited option that Republicans were happy to go along with. By contrast, lifting the filibuster on voting rights would be a lasting change to how the Senate works, and the decision rests entirely on Senate Democrats. “It’s very hard to draw a parallel,” this person said.

Let’s walk through each point.

1. The change to the filibuster for the debt ceiling was temporary. Senators agreed to do it just once. The next time the Treasury Department needs Congress to raise the debt limit so it can pay the nation’s bills, it could face a filibuster. By contrast, to get rid of the filibuster to pass voting rights, Democrats would need to change the rules so that any legislation, now or in the future, regarding voting or election changes is filibuster-proof. That has prompted Sinema to warn that when Republicans get back in power, they could pass national voting restriction bills over Democrats’ objections.

2. The debt ceiling/filibuster debate was technically bipartisan. Three-fifths of the Senate - all 50 members of the Democratic caucus and at least 10 Republicans - voted to waive the filibuster for the debt ceiling. (Not doing so “was something both sides recognized would be catastrophic for a U.S. economy,” the Senate Democratic aide said.) That technically met Manchin’s requirement for tweaks to the filibuster, which is that Republicans are on board with them, too. (Sinema, by contrast, says she won’t support any changes to the filibuster.)

3. It also wasn’t that unique of a workaround. The specific legislative path senators used to tweak the filibuster for the debt ceiling “has been used numerous times by Congress” for other items, said congressional expert Molly Reynolds, a fellow at the Brookings Institution. That’s how budget bills can get past a Senate filibuster (a process known as reconciliation).

4. Manchin and Sinema didn’t have much of a say in the matter. This debt-ceiling deal was a one-time compromise between Senate Democratic and Republican leaders to avoid a default. Rank-and-file senators had little to do with it.

Critics of Manchin and Sinema can and do argue that the two senators are splitting hairs here: The point was that the filibuster was set aside to pass a piece of legislation, and the sky didn’t fall down.

But the debate over what changes to make to the filibuster inherently involves arcane Senate process, so these seemingly little details matter to senators as they weigh big changes to the filibuster.