WASHINGTON -- We need to prepare for the eventuality that the Mueller probe catches President Trump, family members and associates red-handed -- and Republicans in Congress refuse to do anything about it.

This is beginning to look like a possible or even probable outcome. With its cravenness matched only by its arrogance, the GOP is Trump's party now. It no longer has any claim to be Lincoln's.

Witness the cowardly about-face on the subject of Roy Moore's candidacy for the Senate. The party initially took a position in line with its purported values: that a credibly accused child molester, who haunted the local mall seeking dates with teenaged girls when he was in his 30s, is unworthy of the high office he seeks.

But then Trump endorsed Moore -- given that more than a dozen women have accused the president of sexual misconduct, the phrase "birds of a feather" comes to mind -- and Republicans changed their tune. The flow of money from national party coffers to Moore's campaign, briefly interrupted, was resumed. Moore's fitness became a matter that no longer troubled the GOP's moral conscience, or what was left of it; only "the people of Alabama" could decide the difference between right and wrong.

Here is the distinction between our two major parties in 2017: Democratic leaders are forcing Rep. John Conyers, D-Mich., and Sen. Al Franken, D-Minn., both accused of harassing women, to resign. Republican leaders are trying to put Moore, accused of harassing teenagers and molesting a 14-year-old, in the Senate.

Given that context, it is naive to assume that anything special counsel Robert Mueller uncovers will lead Republicans to choose principle over political advantage. Trump boasted during the campaign that he could shoot someone on Fifth Avenue and not lose support. As far as the GOP majorities in Congress are concerned, he may be right.

Ultimately, that's what Trump is counting on. He has been using his Twitter feed to try to create the impression that Mueller -- the straightest of straight arrows, and a lifelong Republican -- is somehow biased against him. Trump's aim isn't so much to pull the wool over the eyes of his base; rather, it's to give House Republicans an excuse not to start impeachment proceedings if and when the time comes.

I realize it seems premature to speak of such eventualities. But if there really was no collusion with Russia to tilt the election in Trump's favor, I wonder why one of the president's lawyers, Jay Sekulow, is arguing that such collusion would not constitute a crime. And if there really was no obstruction of justice, I wonder why another of Trump's lawyers, John Dowd, is arguing that the president by virtue of his office is incapable of obstructing justice.

Perhaps Trump and his family are just getting bad legal advice. I'm wondering who gave Donald Trump Jr. the bright idea to cite attorney-client privilege in refusing to answer the House Intelligence Committee's questions about conversations with his father -- conversations that might bear on both collusion and obstruction.

Neither Trump nor Trump Jr. is an attorney. Conversations between Trump Jr. and his lawyer would be privileged, but not conversations Trump Jr. might have had with someone else while his lawyer was present. And Congress is not compelled to honor attorney-client privilege anyway, though it often does.

At least Trump Jr. gave a reason for his stonewalling. Other administration officials called to Capitol Hill to testify about Trump's possible obstruction -- including Attorney General Jeff Sessions -- have relied on some imaginary privilege not to discuss conversations with the president.

The GOP-led committees investigating the Russia affair could subpoena witnesses, compel them to speak and hold them in contempt if they refuse. I'm not holding my breath.

Those of us who care whether the president worked with a foreign power to influence the election and then actively impeded an FBI investigation do have recourse, however. I've said it before, but it bears repeating: If this Congress refuses to stand up for justice and American ideals, we need to elect a Congress that will.

The Trump presidency poses a challenge beyond politics or policy. Wrongheaded trickle-down tax policy is a matter of arithmetic; it can be corrected by moving numbers around a spreadsheet. But once corruption has become established, it takes Herculean effort to root it out; once respect for the rule of law is lost, it takes generations to restore.

Mueller is doing his job. Ours is to elect Democrats and independents next year who will hold this appalling presidency to account.

(c) 2017, Washington Post Writers Group