President Donald Trump’s allies are looking at his lack of legislative success so far, and they think they’ve found the problem: Senate Majority Leader Mitch McConnell.

On Thursday afternoon, the president — who is currently staying at his golf club in Bedminster, New Jersey on what he says is “not a vacation” — tweeted that McConnell should “get back to work” at passing health care, tax, and infrastructure bills.

Mitch, get back to work and put Repeal & Replace, Tax Reform & Cuts and a great Infrastructure Bill on my desk for signing. You can do it! — Donald J. Trump (@realDonaldTrump) August 10, 2017

This follows a pair of earlier tweets with an even more critical tone toward the majority leader:

Senator Mitch McConnell said I had "excessive expectations," but I don't think so. After 7 years of hearing Repeal & Replace, why not done? — Donald J. Trump (@realDonaldTrump) August 9, 2017

Can you believe that Mitch McConnell, who has screamed Repeal & Replace for 7 years, couldn't get it done. Must Repeal & Replace ObamaCare! — Donald J. Trump (@realDonaldTrump) August 10, 2017

All this was spurred by a statement McConnell himself made in Kentucky Monday, in response to criticisms that his Senate hasn’t done much. “Our new president has of course not been in this line of work before, and I think had excessive expectations about how quickly things happen in the democratic process,” McConnell said.

Trump does not appear to have taken this comment well, and allies of his like Fox News host Sean Hannity have attacked McConnell for it too:

. @SenateMajLdr No Senator, YOU are a WEAK, SPINELESS leader who does not keep his word and you need to Retire! https://t.co/BL4uf7WLM1… — Sean Hannity (@seanhannity) August 9, 2017

The president has also repeatedly tweeted in recent weeks that the Senate should get rid of the filibuster for legislation, so they can pass any bill with a simple majority.

But that’s something McConnell has said the Senate will not do because “the votes simply are not there” and “it would fundamentally change how the Senate has worked for a very long time.”

This showdown may have been inevitable given McConnell’s approach to leading the Senate

The first few months of Donald Trump’s presidency and unified Republican control of Washington have been downright sleepy on the legislative front, and the main reason why is the Senate — and more specifically, the legislative filibuster.

Because the GOP only controls 52 Senate seats, they cannot beat a filibuster, unless they use the special limited budget reconciliation process, or unless they win the votes of at least eight Democrats.

As a consequence, major conservative bill after major conservative bill has passed the House, only to go nowhere in the Senate — hamstringing Republicans’ ambitions for sweeping legislative change. And with the failure of the health bill to even get a majority, the president’s hopes to rack up a series of major accomplishments are fading away.

Now, McConnell did eliminate the filibuster for Supreme Court nominations earlier this year. But that was widely expected, and he appears to be correct that he doesn’t currently have the votes to eliminate the legislative filibuster, despite what Trump might want.

The U.S. Senate should switch to 51 votes, immediately, and get Healthcare and TAX CUTS approved, fast and easy. Dems would do it, no doubt! — Donald J. Trump (@realDonaldTrump) May 30, 2017

Still, McConnell could start trying to lay the groundwork for doing so and making the case for it — and he very conspicuously isn’t. And Trump supporters are noticing.

It’s much harder to oust a Senate Majority Leader than a speaker of the House

Longtime followers of Republican congressional dysfunction may be getting some déjà vu here, recalling how a small group of critics on the right forced House Speaker John Boehner from office.

Yet that’s far tougher to pull off in the Senate, because there’s a difference in how party leaders in each chamber are chosen.

The speaker of the House is elected by the full House of Representatives. Therefore, to win the position on the first ballot, a candidate generally needs overwhelming support from his or her own party (because the opposition party will vote against him or her). As a result, a small group of defectors from the speaker candidate’s own party could block him or her from winning a majority.

The Senate Majority Leader, in contrast, is chosen in a closed-door, partisan election. Only Republicans vote in it. That means that a handful of defectors can’t humiliate McConnell in a leadership election on the floor — instead, over half of his GOP senators would have to turn against him in private.

If Trump supporters really want to oust McConnell, their best chance would probably be in 2020, when he’s up for reelection in Kentucky (where Trump is very popular) and therefore could be vulnerable to a primary challenge. Though McConnell easily fended off a challenge in 2014, the president could potentially make it much harder for him next time.

However, Trump does have some powerful incentives to maintain good relations with McConnell. If he hopes to get his nominees confirmed or get anything at all done legislatively, he really does need the leader. That may be one reason why he just endorsed McConnell’s preferred candidate, Luther Strange, for next week’s special election to fill Jeff Sessions’s Senate seat — leaving two other challengers trying to position themselves as the true Trump candidate in the lurch. But the president is clearly unhappy with the failure of his top legislative priority, and he wants to make his unhappiness known.