Those craving the end of the Trump presidency will hail Jeff Flake and Bob Corker as heroes. They are conservative Republican senators who have turned on their own party’s president, Flake condemning Donald Trump in a powerful, stirring speech in the chamber yesterday, Corker dishing it out in a form his target will understand: an interview followed by a series of sharp, barbed tweets. The latest reads: “Same untruths from an utterly untruthful president. #AlertTheDaycareStaff,” the hashtag being a reference to Corker’s earlier description of the White House as “an adult day care center”.

All that will be heartening to those who understand that, as things stand, the only force that can thwart, impeach or eventually remove Trump is the Republican-dominated Congress. Only if Republicans turn on the president in sufficient numbers will he be imperilled. The optimistic Trump opponent will be hoping that Flake and Corker are the first bulls to break in what will eventually be a Republican stampede.

But there’s a problem. Both men have coupled their criticisms with a decision about their futures, announcing that they will not seek re-election. The good news is that that frees them to oppose Trump trenchantly from now until they formally vacate their seats in January 2019: if they vote with Senate Democrats, they could block Trump altogether, denying him any major legislative achievement. The bad news is that it suggests the price of opposing Trump is your own career.

Play Video 1:24 Senator Jeff Flake announces he won’t seek re-election – video

In which case, few will do it. The lesson of the past 18 months in US politics is that even those Republicans who are privately appalled by Trump’s behaviour will not say so publicly if that risks alienating the Trump base, those Republicans whose votes they need to stay in office. The exemplar is house speaker Paul Ryan, whose reaction to seeing Trump turn on a war widow or praise white supremacists or bait North Korea’s dictator over Twitter is a shrug of the shoulders.

This might be emerging as a key battle line in our politics, and not only in America. In a perceptive Times column on Tuesday, Rachel Sylvester wrote how Westminster had become a hall of mirrors, in which countless politicians were saying and doing things they didn’t believe in – with Theresa May, Philip Hammond and hundreds of Conservative and Labour MPs all pursuing Brexit even though they campaigned for, and still believe in, the cause of remaining in the EU. They do it, in part, because they fear the eventual retribution of the voters if they did otherwise.

But Corker and Flake have suggested a way out of that hall of mirrors. They could no longer defend a president they find indefensible, so they stopped trying. It is an admirable thing, to sacrifice your own political career for the sake of averting what you believe is a disaster – and using your office, while you still have it, to advance your principles, not your prospects. When faced with the twin calamities of Brexit and Trump, more politicians might consider following their lead.

• Jonathan Freedland is a Guardian columnist