PRESIDENT TRUMP is attempting to rewrite the norms of presidential behavior in two fundamental ways in the Ukraine affair. He is claiming the right to directly seek the assistance of foreign governments in pursuing compromising information about his political opponents, even in the absence of any legitimate U.S. investigation. He is also asserting the power to block congressional oversight by prohibiting administration officials from testifying about their official activities, even in private.

These are gross abuses of Mr. Trump’s oath of office. If they are allowed to stand, they will open the way for more offenses in the coming year — including more appeals for foreign intervention in the 2020 election — and they will establish new baselines for future presidents. So congressional Republicans, as well as Democrats, have reason to act forcefully to check Mr. Trump.

So far, they are not stepping up to their responsibility.

Fear-driven Republicans have been enablers of President Trump with their silence, argues Post columnist George F. Will. (The Washington Post)

A handful of Republican senators have objected to Mr. Trump’s calls for Ukraine and China to investigate Joe Biden. Sen. Mitt Romney (Utah) has done so unequivocally. Sens. Ben Sasse (Neb.) and Susan Collins (Maine) have criticized his public appeal to China but have said nothing about Ukraine, where the president’s aides conducted a weeks-long effort to extract a commitment from the new president to investigate a gas company that employed Mr. Biden’s son. Sen. Rob Portman (Ohio) called Mr. Trump’s actions “not appropriate” but criticized House Democrats for having “rushed” to an impeachment investigation.

And Sen. Marco Rubio (Fla.) has criticized Mr. Trump on Ukraine but offered shifting responses on China. First, he said he thought Trump’s appeals weren’t serious. Then he tweeted that “POTUS asking #China to investigate Biden is wrong, but it isn’t going to happen.” He didn’t bother to explain what makes him so sure of that: After all, Mr. Trump has suggested he might raise his request directly with President Xi Jinping, who is seeking leverage over the White House in a trade war.

We’d like to think congressional Republicans would want to learn what, exactly, Mr. Trump did to pressure the Ukrainian government — including whether he withheld vital U.S. military aid that those Republicans overwhelming voted for. The ambassador Mr. Trump appointed to the European Union, Gordon Sondland, likely knows the answer to that question, judging from texts with fellow diplomats that have already been disclosed. But the State Department, apparently acting on Mr. Trump’s wishes, blocked the ambassador from appearing before three House committees on Tuesday. State is also holding on to texts and other documents delivered by Mr. Sondland.

Mr. Trump offered no explanation for squelching the ambassador’s testimony, other than that the House investigation was a “kangaroo court.” A White House letter to Congress cited the House’s failure to hold a formal vote opening its impeachment inquiry. By that standard, any president could henceforth prevent his staff from cooperating with any congressional investigation. Is that the precedent Republicans would set?

The White House’s stonewalling strongly suggests that Mr. Sondland possesses important information about the Ukraine affair. So do other State Department witnesses, including former ambassador Marie Yovanovitch and the current U.S. charge d’affaires in Kiev, William Taylor. Republicans ought to join with Democrats in insisting that the diplomats be allowed to testify to Congress — and Republicans should also make clear to Mr. Trump that his solicitations of foreign governments are unacceptable. Any other course is a betrayal of their own oaths.

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