IT HAS been a year since special counsel Robert S. Mueller III took over the Justice Department’s Russia investigation. Since then, by all appearances, he has performed with professionalism, integrity and remarkable efficiency.

That is not President Trump’s view, of course. The president rants frequently, inappropriately and with no foundation against a supposed “witch hunt.” His complaints only further the impression that he has something to hide. So do the attacks on Mr. Mueller from the Trump claque in the House of Representatives. So did last week’s rather pathetic chiming in from Vice President Pence, who instructed the special counsel, “It’s time to wrap it up.”

In fact, in the space of only one year, Mr. Mueller has secured guilty pleas from, or indictments against, 19 people and three firms, including very senior figures: Mr. Trump’s former campaign manager, Paul Manafort, and his former national security adviser, Michael Flynn. He obtained an indictment against a Russian company that helps illuminate the Russian effort to influence the 2016 election. He has done his work without leaks or drama, even as Mr. Trump and his allies continually slander him and his motivation.

Nor is there any evidence that Mr. Mueller has overstepped proper boundaries of prosecutorial behavior. Deputy Attorney General Rod J. Rosenstein ordered the special counsel to investigate Russia’s 2016 election interference and any matter that “arose or may arise” in doing so. It is only logical that would include Mr. Manafort’s pre-election ties to Russia and the president’s possible post-election efforts to subvert the probe. Any good prosecutor would cover those bases.

Initially, Republican lawmakers praised the selection of Mr. Mueller and emphasized his reputation for honesty. Now that Mr. Trump has decided on a strategy to discredit the investigation, most GOP lawmakers are descending to the level of courage we have come to expect in the Trump era and are staying relatively mute. Polls still show wide public approval of the Mueller probe, but among Republicans Mr. Trump’s attacks are having an effect.

Mr. Mueller deserves more backup from Republicans in Congress — in both word and legislation. They should make clear that he will be given such time as he needs to complete his investigation. Republicans had no objection while independent counsel Kenneth W. Starr investigated President Bill Clinton for nearly five years on issues that were neither as complex nor as important. The American people deserve to learn as much as possible about the Kremlin’s 2016 meddling, how U.S. officials responded to it and whether any U.S. officials cooperated with it.

Some lawmakers have been willing to stick up for Mr. Mueller; the Senate Judiciary Committee approved a bill on a bipartisan basis that protects him from inappropriate termination. So far Senate and House GOP leaders have been unwilling to bring such a bill to a vote. That has left lawmakers scrambling for other options, which could include a bill requiring Mr. Mueller to release a public report on his findings, regardless of his fate. That is less than the bare minimum, but it would be better than nothing.

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