Sen. Rand Paul of Kentucky has a complicated history with President Donald Trump — including attacks lobbed at each other as candidates in the Republican primary and Paul's opposition a White House-backed healthcare bill in September.

On Thursday, the two crossed paths during a signing ceremony for Trump's executive order to unwind parts of the Affordable Care Act, an effort on which Paul and the White House collaborated.

Paul didn't exactly look comfortable.

Here's Paul's reaction as Trump entered the Roosevelt Room:

Watch Rand Paul as President Trump enters the room. pic.twitter.com/yTzTc0Sy73 — Jeremy Art (@cspanJeremy) October 12, 2017

The video of his reaction quickly made the rounds on social media and had even been memorialized in a Twitter Moment on Thursday afternoon.

In the video, Paul offers a few perfunctory claps as Trump greets the audience and begins shaking hands. Though many of the people in the room are smiling, Paul appears stone-faced. He quickly folds his hands together and looks down, shifting his eyes left and right, apparently the only person in the frame whose eyes are not on Trump as the pleasantries continue around him. Noticeably, he never shakes Trump's hand.

Paul opposed Trump's previous healthcare-reform efforts on grounds that they didn't go far enough in repealing the ACA, the healthcare law also known as Obamacare. Paul's opposition to the most recent Senate healthcare bill led Trump to tweet in September: "Rand Paul is a friend of mine but he is such a negative force when it comes to fixing healthcare."

Trump made a winking reference to their differences on Thursday in remarks before the signing.

"I can say, when you get Rand Paul on your side, it has to be positive, that I can tell you," Trump said, drawing laughter from the room. "I was just saying as he's getting up and saying all these wonderful things about what we're going to be announcing, I said, boy, that's pretty unusual. I'm very impressed."