On Tuesday, Senate Majority Leader Mitch McConnell (R-Ky.) declared “case closed” on President Trump and the Russia investigation. On Wednesday, we learned that one of his powerful Senate GOP colleagues disagrees.

As Axios first reported, Senate Intelligence Committee Chairman Richard Burr (R-N.C.) subpoenaed Donald Trump Jr. in recent weeks. In doing so, Burr injected a rather unhelpful subplot into an investigation on which his fellow Republicans are trying to turn the page. While it’s easy to dismiss House Democrats’ nascent subpoena battles with the White House as partisan politics, the fact that a Republican chairman isn’t finished either makes it much more difficult to argue that it’s time to move on.

We don’t know what specifically Burr is interested in hearing from Trump Jr. — just that it pertains to contacts with Russians. Trump Jr. has told Congress (including Burr’s committee in previous testimony) that he didn’t inform President Trump of his meeting with a Russian lawyer who promised dirt on Hillary Clinton. But the Mueller report shows Michael Cohen has called this into question; Democrats have suggested Trump Jr. might have committed perjury. Burr suggested in an interview with The Washington Post this week that this isn’t his focus.

But whatever the underlying issues, the use of a subpoena ratchets this up. It suggests Trump Jr. is resisting a second date, but Burr is insisting. (And judging by the Trump Jr. team’s anonymous attacks on Burr and acting White House chief of staff Mick Mulvaney’s complaints, this is a source of real tension.)

Burr’s move also reinforces a long-standing pattern among Senate Republicans: When they have the power and latitude to run afoul of Trump, they often do — even as their less-potent colleagues have been corralled by Trump and his base.

Burr is the chairman of a committee that prides itself on bipartisanship, and his actions haven’t always toed the party line. He has also said he won’t seek reelection when he’s up in 2022, meaning he need not mind the base quite so much. (His North Carolina colleague, Sen. Thom Tillis (R), by contrast, recently flip-flopped on his past opposition to Trump’s border wall national emergency amid GOP pushback. Tillis faces reelection in 2020.)

The majority of Trump’s most vocal Republican critics in the Senate have chosen to forgo reelection: Jeff Flake, of Arizona, offered his most direct Trump criticisms after announcing he wouldn’t seek reelection in 2018. Tennessee’s Bob Corker was chairman of the Senate Foreign Relations Committee when he began airing complaints about Trump and the White House. Corker later tempered his criticisms, but also opted not to seek reelection. And John McCain traded on his stature as the senior senator from Arizona and the party’s 2008 presidential nominee to offer an occasional check on Trump before his death last year.

We see this to a lesser extent with some current top-ranking Senate Republicans. Senate Finance Committee Chairman Charles E. Grassley (R-Iowa) is currently pressuring Trump to relax his trade war and even presented him with an ultimatum. Senate Appropriations Committee Chairman Richard C. Shelby (R-Ala.) is clashing with Mulvaney over a disaster aid bill. McConnell has even allowed some significant votes rebuking the Trump administration on Saudi Arabia and the president’s declaration of a national emergency on the southern border.

None of these has been a knock-down-drag-out fight between Trump and his own party, but the fact that these situations regularly boil over reinforces the very real concerns within the Senate GOP about how Trump has conducted himself as president. The ones who have the actual power to assert themselves and/or aren’t so terrified of their base have stepped forward on some major issues.

The others, not so much. But it suggests they might, too, if they didn’t feel so powerless and weren’t terrified of the kind of blowback Tillis, Flake, Corker and McCain have received. We may never know the real level of concern that exists within the highest levels of the GOP, but the fact that those who can speak out often do suggests there’s much more beneath the surface.

Burr’s move may be one of the most significant to date. And he’s apparently in for some real pushback from a party base that has largely been convinced the Russia investigation was a “hoax” and a “witch hunt” from Day One. We’ll see how he responds to the pressure.