John Avlon is a CNN senior political analyst and anchor. The opinions expressed in this commentary are the author's own. View more opinion articles on CNN.

(CNN) There's an old saying in American politics that a Democratic crook is just as bad as a Republican crook.

The point, of course, is that we should hold people to the same standard, regardless of political party.

It's a good, solid principle. And it seems almost impossibly quaint, because the search for the truth in the Trump investigations is breaking down along partisan lines.

During Michael Cohen's testimony, Republicans focused more on attacking the witness than questioning his serious allegations. And on CNN's "New Day" this week, Republican Sen. Mike Rounds suggested Trump made hush-money payments to porn star Stormy Daniels to protect his family. "I honestly think this president loves his family," Rounds said.

The new standard seems to be that allegations of lies and obstruction of justice don't matter to Republicans when it's about a president from their party.

We know that this is situational ethics because 20 years ago, some of the same senators were singing a very different tune.

Here's Republican Senate Majority Leader Mitch McConnell talking to CNN in 1999 about impeachment proceedings against then-President Bill Clinton: "The problem is lying under oath and obstructing justice. The subject matter is not what is significant here; it's lying under oath and obstructing justice."

McConnell voted to find President Clinton guilty on both counts -- lying and obstruction of justice.

Sen. Chuck Grassley, who stepped down as chairman of the judiciary committee last year, told CNN the importance of leaving partisanship behind during the impeachment trial in January 1999. "We are miraculously transformed from politicians to people who leave their Republican and Democrat labels at the door...We're there to seek the truth and to find out whether the president is guilty or not guilty, and no stone should be left unturned to make that determination."

Poetic stuff. But today, the search for the truth seems secondary for Senate Republicans.

Trump's former attorney general, Jeff Sessions, took a strong stand against lying from the oval office. After Clinton was acquitted by the Senate on two articles of impeachment in February 1999, Sessions spoke out about the impact it could have on the culture of the country. "It is crucial to our system of justice that we demand the truth. I fear that an acquittal of this President will weaken the legal system by providing an option for those who consider being less than truthful in court," he said

One of President Trump's strongest supporters in the Senate is South Carolina's Lindsey Graham, who was previously considered a member of the maverick posse as one of John McCain's closest friends. When Graham was still a representative in the House, he offered a notably low standard for impeachment. "You don't even have to be convicted of a crime to lose your job in this constitutional republic ... Impeachment is not about punishment. Impeachment is about cleansing the office. Impeachment is about restoring honor and integrity to the office," he said.

Former Sen. Rick Santorum, now a CNN commentator, also was very concerned about the impact of presidential lying at the time: "He lied to protect himself from being prosecuted for a crime. I could think of no other lie that is a more egregious lie... I fear that if this country is confronted with a serious crisis over the next two years, that his ability to marshal the American public behind what he tells them is the truth would be diminished."

When it was all over, a handful of House Democrats voted against Clinton, and several Republicans crossed partisan lines to acquit the president on charges of perjury and obstruction of justice. So while partisan lines did largely hold, there were still some politicians who put their principles ahead of party loyalty.

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There's precious little evidence of that now. One of the first Senate Democrats to publicly condemn Clinton's behavior -- although he did not vote for his removal from office -- was Joe Lieberman, and his independence earned him a spot on the Democratic ticket as Al Gore's running mate in 2000.

Looking at the GOP's approach to Clinton's impeachment trial 20 years ago, it's clear there is now a double standard when it comes to Trump. Hypocrisy used to be the unforgivable sin in American politics. But that seems to be one more casualty of Trump's presidency.