When the Senate Budget Committee meets Tuesday to decide on whether to bring a tax bill to the floor for a vote, one of the Republican party’s most outspoken Trump critics, Bob Corker, will have his first shot at throwing a monkey wrench in the process.

Corker, along with fellow Republican Trump basher Jeff Flake, has expressed reservations about the degree to which the bill adds to the deficit, and reportedly said Monday he would vote no if tax writers can’t provide enough deficit safeguards. The Republicans have only a one-vote margin on the committee so his defection would send lawmakers back to the drawing board.

Bob Corker and Jeff Flake aren’t the only outspoken Republican Trump critics who say they are worried about the deficit, John McCain voted against his own party on tax cuts for the same reason years ago. And their concern is one that won’t go away so long as there are hefty corporate tax cuts, Trump’s top priority. The Republicans can only afford to lose two votes.

These three have not yet said they are a no on the tax bill, but Senate leaders and Trump should be most concerned about their votes for one simple reason. None of the three are up for reelection. To be sure, despite big question marks surrounding the soundness of this tax legislation, the main reason cited by those most optimistic that something will pass is simply that Republicans can’t afford the political backlash of failing to get something done.

But the three publicly anti-Trump Republicans are not the only roadblock. Two others, Senators Steve Daines (Mont.) and Ron Johnson (Wis.), have both come out against the bill, ostensibly to negotiate a better deal for their states. Both say they are optimistic they could get to yes.

There are also a handful of others that join Corker, Flake and McCain in expressing concern about the deficit.

Trump has an uphill battle to fight as he visits Congress today and travels later this week to rally support for the bill. Even if his pitch helps nudge along most of the undecideds, his relationship with the politically immune Corker, Flake and McCain is so toxic he might be more likely to do harm than good. Any Twitter attacks on them might push them to abandon loyalty for their fellow party members in Congress in favor of spiting Trump. Even if he lays off, that may be there plan, nonetheless.