President Trump probably won't be impeached. Doing so would require the Republican-controlled House of Representatives to hold hearings to determine Trump's crimes -- a task the House GOP is unlikely to pursue (party over nation, etc). Then they'd have to draw up articles of impeachment and call a vote on the articles. If Trump is impeached, it'll then require 67 Republican senators to line up in support of convicting Trump of high crimes.
Unless Trump becomes unbearably toxic -- like 20 percent approval toxic, it's unlikely the Republicans will attempt to oust their own party's de facto leader. As for achieving a Democratic majority following the 2018 midterms, the map is extremely challenging. On the Senate side, the Dems would need to hold every one of their 25 blue seats that are up for re-election, plus they'd have to win three of the eight contests in which Republicans are up for re-election. In other words, there'd have to be an historic sweep of the Senate in the midterms, accompanied by the Democrats walking away with a 20-seat net gain in the House. Another challenging task, especially coupled with both gerrymandering and the reality that Democrats don't often turn out for midterms.
So, what's the most likely scenario for deposing Trump? (I underscore "most likely" because, at this point, it involves Republicans doing the right thing.)
The 25th Amendment might work, but it's loaded with loopholes and would only serve as a temporary stop-gap. Plus, the GOP would have to make a serious case that Trump is mentally unfit.
The only viable option is for Republican leadership to approach Trump and to ask him to step down for the good of the nation. The Nixon 1974 gambit. This is the most likely scenario -- far more likely than a lengthy agenda-killing impeachment. Leadership would simply ask Trump to step down. Of course, he'll refuse -- and at his own peril. Trump isn't politically savvy enough to understand the subtext of the meeting: resign or we'll make your presidency more of a living hell than it already is. If I was Paul Ryan, I'd be prepared for Trump to stubbornly remain in office and, if that's the case, I'd be ready with a thorough plan for undermining and handcuffing Trump until he can no longer serve.
The leaks would be more like geysers. Suddenly, the tsunami of bad news emanating from the White House would be joined by embarrassing syllabus of leaks about Trump's businesses, his taxes, his personal issues and so on. He'd become a political dead man, shuffling around the White House without any backing from the other end of Pennsylvania Avenue.
If he continues to linger, I'd assemble a dream ticket to primary challenge him in 2020. Let's say, a Kasich-Rubio ticket -- announcing both the top and bottom of the ticket from the outset. Imagine you're a Republican, struggling to punch your way through Trump's fusillade of insanity as well as an activated left and a coalition of anti-Trump Republicans, and suddenly here comes this golden ticket providing a welcome breath of fresh air.
Paul Ryan and Mitch McConnell would do well to have an insurance policy like this ready to roll, especially knowing how Trump could explode the entire GOP, sending Republicans scurrying into the private sector after a shellacking in 2018 and 2020.
Whatever the scenario might be, Trump will have to be dragged out of the White House, and the Republicans don't have the votes, the political capital, or the balls to impeach him.