FROM THE beginning, the Trump presidency posed a unique challenge to the American system of government and, indeed, to the political theory upon which it was built. By separating the legislative, executive and judicial powers among three coordinate branches and providing means for each branch to check or balance the others, the framers of our Constitution sought to protect liberty from the various menaces posed by human nature itself. Ambition, recklessness, greed, incompetence and excessive partisanship — any or all of these might gain a foothold in one part of government, but as long as countervailing forces existed they could be prevented from ruining the whole thing.

President Trump’s rise tests the American system because he was elected on the strength of radical protest against it — the claim that it’s all “rigged” — and because his party dominated Congress as well as the White House. The erratic disrupter-in-chief came to power with a political escort of enablers. And so any hope that checks and balances would work to constrain Mr. Trump’s worst impulses hinged, in part, on the willingness of Republicans in Congress to act in defense of values higher than short-term political advantage, or at least to interpret their short-term political interest as requiring them to counter Mr. Trump.

This week brought the most encouraging signs yet that members of the GOP are indeed willing to behave as the framers would have had them do. Republicans in Congress voted with Democrats in overwhelming, veto-proof, numbers to pass a Russia sanctions bill that constrains Mr. Trump’s ability to indulge his strange sympathy for Vladi­mir Putin’s despotic regime. And they pushed back against Mr. Trump’s increasingly aggressive and increasingly bizarre attacks on Attorney General Jeff Sessions, which were implicitly attacks on independent counsel Robert S. Mueller III. Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) made it clear that his committee would give the president no opportunity to appoint a replacement to Mr. Sessions if he tried to fire him. Meanwhile, Sen. Lindsey O. Graham (R-S.C.) announced a joint effort with Democrats to legislate a bar to firing Mr. Mueller, an eventuality which, he said, would mark “the beginning of the end of the Trump presidency.”

To be sure, Mr. Graham’s attempt to impose judicial review on a president’s firing of a special counsel might fail in practice, for any number of technical legal and constitutional reasons. We admire the spirit behind it, however, just as we support Republican efforts to rein in Mr. Trump’s worst instincts on Russia policy, as well as the efforts of the Republican-chaired Senate intelligence committee to investigate the Russian nexus to the 2016 campaign. In the minds of many of Mr. Trump’s harshest critics, especially Democrats, anything short of impeachment constitutes Republican submission to an unfit and illegitimate president. What GOP lawmakers’ first serious steps toward checking and balancing Mr. Trump showed this week, however, is that there is a middle ground, which members of his own party are no longer afraid to explore.