Congressional Republicans are likely to push through their tax policy legislation next week — but they should be getting a little nervous.

Here’s the most likely scenario for the next few days, one that should comfort Republicans and President Trump. House and Senate Republicans already have the outlines of a compromise bill that would reconcile the separate versions that the two chambers passed individually. Because those bills were fairly similar (centered on reducing the corporate tax rate and getting rid of most deductions for individuals), I don’t expect many members who supported the bills in the House and the Senate to oppose them now.

But let’s say there are some defections. How much trouble would the tax bill be in?

In the House, Republicans have plenty of margin for error: 226 of the current House Republicans voted for the tax legislation, compared with just 13 who opposed it. So Republicans could afford nine more defections and still get the bill approved. And they likely won’t even need that cushion: There is no sign of a major emerging opposition to the tax bill in the House, even though it remains unpopular with the public.

The Senate is trickier. Tennessee’s Bob Corker was the only Senate Republican to oppose the tax bill when the chamber voted on its version of the legislation. So the bill was approved 51-49. Republicans can afford at least one more defection and still pass this legislation 50-50, with Vice President Mike Pence casting the tie-breaking vote. Two more defections would kill it.

And in recent days, there have been rumblings of another potential defection.

Florida’s Marco Rubio, who voted for the Senate version of this legislation, said on Thursday that he opposed the current bill emerging from the House and Senate conference. The compromise legislation is expected to include a $2,000 tax credit for each child a family has. But families that don’t have enough income to owe federal income taxes would get only a $1,100 credit per child. Rubio says designing the child tax credit that way leaves lower-income families not receiving much benefit, so he wants to change the bill so families that pay no income taxes get something close to $2,000 per child. The office of Utah Sen. Mike Lee, who has allied himself with Rubio on this tax credit issue, has said he is now undecided on the legislation, even as he backed the original Senate version as well.

So why do I think Republicans should be optimistic? Because it’s hard to see Lee and Rubio — two Republicans whom everyone expects to run for president in the future (for a second time, in Rubio’s case) — tanking one of the party’s biggest legislative priorities. Rubio in particular.

The Florida senator hasn’t exactly been a maverick. He’s voted against the Trump administration — i.e. voted “yes” when Trump opposes a bill or “no” when Trump supports it — only two times this year. Those two votes came on different Senate provisions that, in effect, limited Trump’s ability to lift sanctions imposed against Russia. Both passed overwhelmingly. (And Trump, while criticizing it, still signed a version of one of those bills into law.) By FiveThirtyEight’s measure, Rubio ranks third among Republican senators in voting with Trump’s agenda more often than we’d expect given the politics of his state, which the president won by just 1.2 percentage points in 2016.

How often GOP senators vote with Trump vs. expectations Difference between a member’s actual and predicted Trump score SCORE MEMBER STATE TRUMP PREDICTED DIFF Cory Gardner CO 94.4% 44.5% 50.0 Dean Heller NV 90.7 49.2 41.5 Marco Rubio FL 96.2 56.0 40.2 Ron Johnson WI 92.6 55.7 36.9 Patrick J. Toomey PA 90.4 55.0 35.4 Richard Burr NC 96.2 60.9 35.3 Thom Tillis NC 96.3 61.6 34.7 Susan M. Collins ME 81.5 48.1 33.3 Johnny Isakson GA 94.7 63.4 31.4 David Perdue GA 94.3 64.0 30.4 Jeff Flake AZ 90.2 60.5 29.7 John Cornyn TX 96.3 71.6 24.7 Rob Portman OH 92.6 70.1 22.5 John McCain AZ 82.7 60.9 21.8 Ted Cruz TX 92.6 71.6 21.0 Chuck Grassley IA 92.6 72.2 20.3 Joni Ernst IA 90.7 72.2 18.5 Dan Sullivan AK 96.1 79.7 16.3 Tim Scott SC 94.4 79.3 15.1 Thad Cochran MS 96.3 83.3 13.0 Roger F. Wicker MS 96.3 83.3 13.0 Orrin G. Hatch UT 96.3 83.5 12.8 Roy Blunt MO 96.3 84.1 12.2 Bill Cassidy LA 96.3 85.0 11.3 Pat Roberts KS 96.3 85.8 10.5 Lindsey Graham SC 88.9 79.3 9.6 Jerry Moran KS 94.3 85.6 8.8 Todd Young IN 92.6 84.6 8.0 John Kennedy LA 92.6 85.0 7.6 Lisa Murkowski AK 86.8 79.6 7.2 Steve Daines MT 92.6 85.6 7.0 John Boozman AR 96.3 89.7 6.6 Mitch McConnell KY 96.2 90.7 5.5 John Thune SD 96.3 90.9 5.4 Mike Lee UT 88.9 83.5 5.3 Lamar Alexander TN 94.2 88.9 5.3 Richard C. Shelby AL 94.4 90.1 4.3 John Hoeven ND 96.3 92.3 4.0 Deb Fischer NE 92.6 88.8 3.8 Mike Rounds SD 94.4 90.9 3.6 Tom Cotton AR 92.6 89.7 2.9 Luther Strange AL 90.5 88.2 2.3 James M. Inhofe OK 94.4 92.4 2.0 Shelley Moore Capito WV 94.3 92.8 1.5 John Barrasso WY 94.4 93.0 1.4 Mike Crapo ID 92.5 91.4 1.0 James Lankford OK 92.6 92.4 0.2 Michael B. Enzi WY 92.6 93.0 -0.4 Ben Sasse NE 88.7 89.1 -0.5 James E. Risch ID 90.7 91.5 -0.8 Bob Corker TN 84.9 89.1 -4.2 Rand Paul KY 84.3 90.7 -6.4

Instead, what seems likely to happen over the next few days is that Republican leaders make small changes (say, make the tax credit $1,200 per child for those who don’t pay income taxes) and Rubio backs the bill. (There are already indications that such a compromise will be achieved.) Or Lee embraces the bill, Rubio votes no, and Republicans still get the legislation to Trump’s desk.

That said, there are two other plausible scenarios where things fall apart for Republicans next week.

Marco and Mike go maverick — Remember, during the Obamacare repeal process, Arizona’s John McCain gave an impassioned speech about the Senate working more efficiently. Two days later, after a bizarre process that featured Republicans in the Senate settling on a “skinny repeal” that many GOP members hated, McCain voted it down. Senate Republican leaders had basically made a mockery of his concerns, and McCain followed through on them.

Similarly, during the process to pass the tax bill in the Senate, Rubio and Lee pushed a provision that would increase the child tax credit for families that don’t owe much in income taxes. To fund that, the duo proposed that Republicans, instead of lowering the corporate rate from 35 to 20 percent, reduce that tax to 21 percent. Senate Republicans voted down the idea, in effect saying that a corporate rate of 20 percent was more important than Rubio and Lee’s tax credit.

The emerging Senate-House compromise tax bill does put the corporate rate at 21 percent, but as a way to fund a reduction of the top individual tax rate (to 37 percent, instead of the current 39.6 percent). So Republicans rejected Rubio and Lee’s idea of giving more money to low-income families but accepted a proposal to cut taxes for upper-income people. This is a blunt dismissal of Rubio and Lee’s views, and the Florida senator is publicly complaining about it.

Republican leaders have basically called Lee and Rubio’s bluff. Maybe they’re not bluffing.

McCain misses the vote — McCain is suffering from brain cancer and has not been on Capitol Hill in recent days. He favors this tax bill, and GOP leaders his office have said as suggested he will be able to vote next week. But if he can’t vote, that leaves only 99 senators.

So that means that a 49-50 vote kills the bill. Forty-eight Democrats, Corker and just one additional Republican can block it. If McCain is not around, Lee or Rubio would have a ton of leverage.

It’s important to emphasize that Republicans are unlikely to give up on this legislation even if it doesn’t pass next week. But Congress is expected to leave for a recess on Dec. 22 (next Friday) and not return until Jan 3. When they come back, Sen.-elect Doug Jones, Democrat of Alabama, is likely to have been sworn in, further narrowing the Republican Senate majority. Members in the House and Senate may return wary of voting for this bill, after being in their districts over the holidays and being reminded about its unpopularity.

So Republicans should probably hurry up and finish this week if they want this tax overhaul to pass. I think they will.