“Never wrestle with a pig,” says the old maxim. “You both get dirty, and the pig likes it.” That’s the risk facing Democrats who hope to impeach and remove the president or defeat him at the polls. But sometimes, pig wrestling is a task that cannot be avoided.

Donald Trump is not typical of politicians or human beings. When accused of wrongdoing that they actually committed, most of them would either deny the behavior or admit and repent of it. Trump is unusual because his defense is not that he didn’t do it, or that it wasn’t bad, but that there are no standards of good and bad that mean anything.

The evidence is abundant that he tried to extort the president of Ukraine to come up with incriminating information about Joe Biden by withholding $391 million in security aid.

Presented with this information, Trump first insisted the news stories were “just another political hack job.” But before long, he essentially confessed to all the elements of the case against him.

He acknowledged withholding the aid. He said he brought up Biden in his July call with the president of Ukraine. On Monday, he followed a denial with an admission: “There was no pressure put on them whatsoever. But there was pressure put on with respect to Joe Biden. What Joe Biden did for his son, that’s something they should be looking at.”

The more revealing statement made by Trump, however, was this: “I did not make a statement that ‘you have to do this or I’m not going to give you aid.’ I wouldn’t do that.”

The White House transcript of the call, released Wednesday, confirmed all this. Trump did urge the president to investigate Biden. Trump also made a particularly pointed remark: “The United States has been very, very good to Ukraine.”

The episode is Trump behaving like a mob boss. He withholds aid; he asks for a favor; but he doesn’t say the aid is contingent on the favor. He doesn’t have to. The Ukrainian president wasn’t born yesterday.

Neither were the American people. But Trump’s response to evidence of his misconduct and malfeasance doesn’t really depend on getting Americans to believe he is innocent. It depends on getting them to believe that no one is. It’s a nasty, dishonest, corrupt world, he suggests, and his unsavory character makes him ideal for navigating it.

Trump wants us to think that such rivals as Biden and Hillary Clinton are even sleazier than he is. He wants us to either suspend our standards of acceptable conduct or else conclude that no one in presidential politics meets those standards. It’s a strategy aimed both at foiling Congress from impeaching and removing him and at winning reelection.

This approach worked to damage Clinton enough for him to win in 2016. It’s possible it will inflict some damage on Biden. But there are reasons to think it won’t be as effective this time.

One is that his claims about Biden are thin. The then-vice president’s 2016 calls for the resignation of Ukraine’s prosecutor general came as part of a broad effort by Western governments to combat corruption, not as a rogue effort to benefit his son, who was on the board of a Ukrainian natural gas company — and who has not been charged with any wrongdoing.

Another is the public harbor so much distrust of Trump that his accusations will lack credibility with anyone except die-hard Republicans. It’s not just that most people disapprove of his performance in office. A May Quinnipac poll found that by a 2-to-1 margin, Americans think he committed crimes before becoming president. Among independent voters, it showed, 54 percent trust the media to tell the truth more than they trust Trump, with only 29 percent taking the opposite view.

With Clinton, Trump had the advantage of identifying her with her husband, whose record included many instances of deceptive, unethical behavior — including his affair with Monica Lewinsky, his habit of giving false testimony and his pardon of Marc Rich, whose former wife was a big donor to the Clintons.

Biden, by contrast, is most closely associated with Barack Obama, who was far more popular than Trump has been and whose administration, unlike this one, was largely free of indictments. In a Pew poll last year, more people rated Obama the best president of their lifetimes than any other president.

In the coming months, Biden and Democrats in Congress can expect to be soiled by Trump. But even in a muddy pen, Americans will have no trouble telling which one is the pig.