With the announced resignation of Defense Secretary Jim Mattis on Thursday and the release of his letter detailing criticisms of President Trump, many Republican leaders have been more willing than usual to express disappointment in the president. Granted, “more willing than usual” includes things such as Senate Majority Leader Mitch McConnell’s hand-wringing statement on Mattis’s departure. But still, some distance from the president that we haven’t seen before.

It should be particularly worrisome to Trump that McConnell (R-Ky.) offered any words of criticism at all. After all, should the House vote to impeach Trump on whatever grounds it deems fit, it would come down to the Senate to determine whether he remains in office. If the Senate turns on Trump, that could be very bad news for his presidency.

But there is very little for Trump to worry about.

As of this writing, in the lame-duck session of the 115th Congress, the Republicans hold 51 seats in the Senate. The Democrats (including the two independents who caucus with the party) hold 49. If we look at the distribution of those seats relative to the 2016 election results, we see, unsurprisingly, that most of the seats held by Republicans are in states won by Trump two years ago.

But, of course, we had an election last month. The results of that election were to shift the Senate slightly more toward the Republicans, as Trump’s party won four Democratic seats and the Democrats won two Republican ones.

The result? In the next Congress, there will be 53 seats held by Republicans. Only two of them will be representing states that Hillary Clinton won in 2016. Eleven are from states that Trump won by five points or fewer.

To remove Trump from office, the Senate would need a two-thirds majority. In other words, the 47 Democrats would need to be joined by 20 Republicans for a total of 67 votes. And that’s the main sticking point: Are there 20 Republicans who would vote to oust their party’s president?

There are 20 Republicans who represent states that Trump either lost or won by 15 points or less. Could all of them be persuaded to vote to remove him from office? To make the point clearer: Two Republicans are from the state that Trump won by 15 points, Alaska. Would, say, Sen. Dan Sullivan (R-Alaska) vote with the Democrats on removal?

One thing we saw in the 2018 elections was a broad swing to the left from the 2016 results. On average, Senate contests last month were 10 points friendlier to the Democrats than the 2016 results were. Only a few states voted more heavily Republican, including Utah, which was skeptical of Trump but embraced Sen.-elect Mitt Romney.

If all of those Republican senators were up in 2020 and the same swing occurred, every senator from a state that voted for Clinton or backed Trump by 10 points or less could be at risk, making them, theoretically, vulnerable to being persuaded to buck Trump. But that’s only 18 senators.

What’s more, that premise is wrong. First, because it’s not fair to assume that the swing will be the same in 2020 as it was in 2018. Trump himself is slated to be on the ballot in two years' time, and, he would argue, that will bring out Republican base voters who may have stayed home this year.

But, more important, not all of those senators are up for reelection in 2020. Twenty-three are, which is no small thing, but many of them are from states that Trump won handily last time around.

There’s another underlying question: Who’s more likely to vote to buck Trump, someone who’s on the ballot or someone who isn’t?

Consider what happened in the months leading up to President Richard Nixon’s resignation from the White House. As the Watergate investigation unfolded, his support from all Americans began to erode, dropping from more than 60 percent approval shortly after his reelection in 1972 to the low 30s by mid-1973.

By the time he resigned, only about a quarter of the country approved of his job performance. While half of Republicans still did, their support for the president waned along with the overall population’s.

Polling at the time offered a mixed assessment of whether Nixon should resign, but the weakening of support within the Republican base as midterm elections loomed, combined with revelations about Nixon’s involvement in illegal activity, quickly made his position untenable.

Trump, facing a broad array of criticisms and questions, has seen no similar erosion. He started with low overall approval and high Republican approval, and both of those things have held. In fact, his approval among Republicans has actually improved a bit over his presidency.

Hence the question about who would vote to oust Trump, someone on the ballot or someone who isn’t. If Trump holds as much support among Republicans moving forward — despite Mattis’s departure, despite Trump’s implication in campaign finance violations or despite who-knows-what might come down the pike — Republicans on the ballot will be inclined to stick with him, at least through the GOP primaries. It may be those Republicans who are in seats that aren’t up in 2020 who feel empowered to break with the president.

As it stands, it’s hard to see how 67 votes could be cobbled together. Trump remains popular with his party, and the Republicans hold a majority of Senate seats. As Nixon could attest, things can change quickly. But they haven’t started changing much yet.