The openness and intensity of the opposition to President Trump among Republicans in Washington seem to have jumped up a level, at least anecdotally: Iowa Sen. Chuck Grassley and others have criticized him for suggesting that he would fire Attorney General Jeff Sessions; Sen. John Thune of South Dakota and others have openly said that the Senate won’t keep pushing to pass an Obamacare repeal even if Trump wants them to; and Sen. Jeff Flake of Arizona sharply attacks the president in his new book.

But did you notice a pattern in the examples above? They all involve senators, not House members. The uptick in public pushback against Trump seems — we don’t have great data yet on this question — to be coming more from one chamber on Capitol Hill than the other. Here are some potential reasons:

1. Duh, re-election

In trying to get to the bottom of why the Senate seems more outspoken against Trump than the House, I reached out to a handful of political scientists who study Congress, along with a few longtime Capitol Hill staffers. Several of them noted that one big factor could be that all GOP House members are up for re-election next year and that most GOP senators (43 of the 52) are not. The theory here is that being anti-Trump would hurt a Republican senator or representative with the GOP base and could lead to either a primary challenge or depressed support in the general election. Since all House members are facing re-election, more are shying away from open criticism of the president.

This theory fits some of the evidence we have so far. Sens. John McCain and Lisa Murkowski — two of the Republicans who have taken the most high-profile anti-Trump moves so far — both won re-election last year, which means they won’t face voters again until 2022. Along with those two, the other “no” vote on the version of the Obamacare repeal legislation that Senate GOP leaders had hoped to pass was Maine’s Susan Collins, who is not up for re-election until 2020, the same year that South Carolina Sen. Lindsey Graham, another regular Trump critic, would run for another term.

“Senators both serve longer terms and have broader, state-based constituencies that mean their incentives vis-à-vis their party might be different than House members,” said political scientist Molly Reynolds, who is a fellow at the Brookings Institution and specializes in Congress.

In contrast, Nevada Sen. Dean Heller, who is up for re-election in 2018, criticized the Obamacare repeal effort early in the process but voted for the final product after Trump suggested that a “no” vote could imperil Heller’s support from the party. (Flake’s decision to release a book this year that is critical of Trump is surprising not only because he is up for re-election in 2018, but also because he is facing a GOP primary challenger.)

In an article published earlier this year, FiveThirtyEight contributor David Wasserman argued that GOP Senate candidates who publicly withdrew their backing of Trump in 2016 suffered electorally. Looking at local-level data from November’s general election, Wasserman found that these candidates underperformed in counties where Trump was strongest. Through a series of surveys of Republican voters that they conducted this year, political scientists Alexandra Filindra and Laurel Harbridge-Yong argued that there is clear evidence that opposing Trump would hurt a Republican in a primary.

I would add one caveat to the general idea that opposing Trump is not smart politics: Most Republican House members seem to be more popular than Trump in their districts, so it’s not clear that they have much to fear from annoying the president. According to research from Daily Kos, in 178 of the 241 House districts that Republicans won in 2016, the GOP candidate won by a larger margin than Trump did. In only 34 of the 241 districts that Republicans carried did Trump win by more than the GOP House candidate did. (In the other 29 races, there was no Democrat on the ballot.)

2. Senators may dislike Trump more than House members do

By last October, near the end of the presidential campaign, 14 of the 54 then-sitting GOP senators (26 percent) were refusing to back Trump, according to Vox. In contrast, just 29 of the 247 House members the GOP had at that time refused to back Trump (12 percent).

Maybe Republican senators are speaking out against Trump now because a large bloc of them didn’t really want him to be president in the first place. Of that 14, two lost re-election, but 12 are in the chamber now: Idaho’s Mike Crapo, Colorado’s Cory Gardner, Utah’s Mike Lee, Ohio’s Rob Portman, Nebraska’s Ben Sasse, Alaska’s Dan Sullivan, Collins, Flake, Graham, Heller, McCain and Murkowski.

So, what do the senators in this more anti-Trump group have in common? Honestly, I’m not sure. I don’t think the answer is ideology — the attempt to repeal and replace the Affordable Care Act in the Senate was challenging in part because Collins (more liberal) and Lee (more conservative) didn’t agree on much. To its credit, Vox noticed something back in October: The Trump-skeptic wing of the Senate GOP is almost totally outside of the South, a solidly Republican region. Of this group of 12, only Graham is from the South.

But I’m not sure that these members are from outside the South explains their opposition to Trump, who is fairly popular in the South but also in states like Alaska, Idaho and Nebraska, where some of the president’s Senate critics are from.

3. The Senate provides more high-profile opportunities for criticism

The House is not involved in approving executive branch nominations. So it’s logical that senators, who would have to approve a new attorney general, appear to have publicly weighed in more than House members on the issue of whether it would be proper for Trump to replace Sessions.

And we may also be capturing a media effect here: Senators are more well-known, so journalists may choose to ask them their views on issues and quote them more than rank-and-file House members. It could very well be that if all House members were interviewed as often as Graham and McCain are, we would have a deeper sense of House members’ grievances about the president.

Remember that in one of the most high-profile anti-Trump moves of the year, most GOP House members overwhelmingly voted to limit the president’s ability to lift sanctions on Russia, just as the Senate did.

“Senators have much bigger microphones and audiences,” said Matt Glassman, a political scientist and co-editor of the 2016 book “Party and Procedure in the United States Congress.”

4. The House may be more aligned with Trump on issues

Trump is not an establishment Republican, a tea party-type or a moderate. So there is not really a wing of Republicans in either the House or the Senate that is Trump-like.

But if you were looking for groups that were generally aligned with Trump on issues, you could find two — both of which are in the House. The House Freedom Caucus is much more focused on reducing the size and role of government than Trump is, at least judging by Trump’s public comments. But in terms of viewing Washington as fundamentally broken, the Freedom Caucus (and a few conservative senators, like Ted Cruz of Texas) is closer to Trump than other parts of Congress are. That probably explains why, after some initial tension over health care policy, some Freedom Caucus members have aligned themselves with Trump.

The House also seems to have something of a Fox News-wing — a group of members who, amid some of Trump’s struggles, want to turn the attention back to the foibles of the Obama administration and Hillary Clinton, as the conservative cable network regularly does. The GOP-controlled House Judiciary Committee is urging the Justice Department to appoint a second special counsel, in addition to Robert Mueller, who is investigating Russia’s interference in the 2016 election and alleged ties between Russia and the Trump campaign. According to a letter the committee sent to top officials at the department, the additional special counsel should investigate Hillary Clinton, former Attorney General Loretta Lynch and former FBI Director James Comey, among others.

Republicans on the Senate Judiciary Committee are not pushing for such a special counsel. In fact, GOP Sen. Thom Tillis of North Carolina, who is a member of the committee, introduced a bill this week that seemed to be aimed at protecting Mueller from being fired by Trump. The legislation would allow for judicial review of the firing of any special counsel, with a three-judge panel authorized to restore the counsel if he or she were fired for reasons determined by the judges to be invalid. Graham, also on the Judiciary Committee, introduced a similar provision.

“This is a pivotal moment within the Republican Party, where the commitment to checks and balances is overriding party politics with the White House,” said Katherine Kidder, a political scientist who works at the bi-partisan Center for a New American Security in Washington. “It’s about congressional-executive relations as much or more than partisan relationships.”

5. Paul Ryan may support Trump more than Mitch McConnell

House Speaker Paul Ryan, perhaps compensating for his disavowal of Trump during the campaign, regularly defends the president now. For example, while multiple GOP senators, including Majority Leader Mitch McConnell, spoke out in support of Sessions when the attorney general was under fire from Trump, the House speaker argued that the president could hire and fire whomever he wanted in the executive branch. The episode was typical of the two leaders’ general stances toward Trump: McConnell rarely criticizes Trump directly, but often does not outright defend the president either; Ryan has been quicker to take the president’s side.

The differing approaches could be because the party leaders are following the desires of their members — so Ryan is more pro-Trump because House members are and McConnell is less pro-Trump than Ryan because Republicans senators are more willing to criticize the president.

But it also could be that McConnell is setting a tone that senators should challenge Trump when they see fit and Ryan is not. The limits on Russia under the sanctions legislation were first pushed through in the Senate, effectively forcing the House into action. Party leaders have a lot of control over which legislation goes to the floor for votes, and McConnell put the anti-Trump Russia bill up for a vote first.

So what does this mean for Trump and the Republican-controlled Congress? Well, if there are a bunch of senators who just don’t like Trump and never will and don’t have any political reason to support him (because they aren’t up for re-election soon), this is a potentially intractable problem for the president. On the other hand, if senators like Graham are open to working with Trump, despite not wanting him to be president in the first place, there would seem to a path forward for the president and his new chief of staff, John Kelly: build closer ties with McConnell, abandon any notion of firing Mueller or Sessions and move off of the Obamacare repeal effort that the senators don’t want to continue. We’ll see if Trump takes this path or decides, as some of his tweets have hinted, that he wants to bash Senate Republicans, not court them.