In an interview with "Axios on HBO," Sen. Lindsey Graham, one of President Trump's most vital allies on Capitol Hill, opened the door to changing his mind on impeachment if there turns out to be what he considers a quid pro quo.

Why it matters: Graham was a fiery House prosecutor during the 1998 impeachment trial of President Clinton. Now that Graham is in the Senate, he'll vote to acquit — or remove — Trump if he's impeached by the House.

"Sure. I mean ... show me something that ... is a crime," Graham told Axios' Jonathan Swan. "If you could show me that, you know, Trump actually was engaging in a quid pro quo, outside the phone call, that would be very disturbing."

"As to asking China to look into Biden, that was stupid. ... Bad idea. That didn't last very long. I think that's a frustrated Trump."

But Trump's Ukraine call isn't impeachable on its own, Graham said: "I've read the transcript of the Ukrainian phone call. That's not a quid pro quo to me."

The interview was Tuesday, before acting White House Chief of Staff Mick Mulvaney gave Thursday's "quid pro quo" briefing.

was Tuesday, before acting White House Chief of Staff Mick Mulvaney gave Thursday's "quid pro quo" briefing. Graham's spokesperson, Kevin Bishop, said Friday that Graham still has not heard or seen anything that he deems impeachable.

The big picture: Trump's loosening hold on Graham reflects the mess the president has created for himself in the past two weeks.

At the very time he needs a Republican fortress against impeachment, GOP lawmakers are furious at him over his rash pullback in Syria.

he needs a Republican fortress against impeachment, GOP lawmakers are furious at him over his rash pullback in Syria. In the interview, Graham called Trump's abandonment of the Kurds "dishonorable" and a "sh*tshow."

Graham said he's changed his view of Trump's character since opposing him during the 2016 primaries:

"I've got to know him, and I find him to be a handful," Graham said. "I find him to be an equal opportunity abuser of people. But at the end of the day, he can be very charming and be very gracious, and I'm judging him by his conduct."

Graham, a Trump golfing companion, said he continues to support Trump's presidency because he's "a Republican. I like his domestic policies. So you play the ball as it lies."