Currently, GOP holds a plurality in the Senate and a majority in the House. The House votes to impeach; if that vote passes, impeachment proceedings are held in the Senate.
Two Presidents have been impeached (Andrew Johnson in 1868, Bill Clinton in 1998) and neither was removed from office. Ironically enough, Johnson was impeached by his own party, the GOP, which didn't like him very much.
Because the GOP is running the House, it would be a grave, grave error to hold a vote to impeach Trump (unless he was manifestly guilty of something heinous. Firing Comey isn't it). That vote only comes up if the GOP lets it, and the GOP would be shooting itself in the foot if it did.
Last year the big worry among Trump supporters was that the GOP would nullify the results of the primaries and instead select a candidate from among its establishment. We warned the GOP then that doing something of that nature would mean the end of the party; the consequences of impeaching Trump would be no less dire.
The people on the left who are calling for Trump's impeachment don't seem to realize that they'll get Mike Pence as President if their wish is granted. The GOP doesn't care, since Pence is one of them; but if Trump were impeached next week, and removed the week after, the GOP would lose big in the 2018 midterms and Pence himself would not win the election in 2020 unless he governed further right, and with more fervor, than Trump has. He would have to be more Trump than Trump is to win that election.
I add that caveat because what GOP voters want most is victory over the creeping stain of leftism. They want America to be great again; they want Obamacare repealed, they want illegal immigration curtailed, they want the LGBTWTFBBQ lobby to STFU, use the right bathrooms, and leave them alone. They want as much government as possible out of their lives. They want a safe and free country like we had as recently as 1975.
But Pence winning election by governing that way would not carry over to the GOP in general. It would be a rout. The midterm GOP vote would be split and Democrats would retake the legislature. Pence would not win 2020 because he would spend the remainder of the term fighting Democrats and getting nothing done. We might get another right-wing President in 2024 but he would not be a Republican; he'd be from a new party, one that arose from the wreckage of the GOP.
A party cannot survive while ignoring the will of its voters. Hillary lost (despite Democrat cheating etc) in part because the Democrats ignored the expressed will of its voters. The majority of Democrat voters wanted Bernie Sanders. Enough of the Sanders voters stayed home on election day that Trump won.
I'm reminded of the enthusiasm with which I attended my first GOP caucus, in Cedar Rapids, in 2000. Prior to that my participation in politics was simply to hold my opinions, listen to Limbaugh, and vote GOP. I voted for Dole in 1996. In fact--
Found that while cleaning stuff out of the garage, in one of my boxes of papers. Didn't put it on my car because it was new (bought just that January, the green Escort in fact) but I'd picked it up intending to tape it up in my rear window, until I realized some commie-lib would probably vandalize my new car.
But the caucus in 2000, that was a new experience for me, and it was kind of exciting to be involved in the selection of a candidate instead of just filling out a voting form.
That enthusiasm didn't last very long. I was kind of naive, thinking that Bush would govern conservatively. Then 9/11 came, and that set in motion a chain of events which left me living in Illinois again before the next presidential election. There wasn't going to be a primary, anyway, not with Bush being a wartime President, and to be honest while I approved in general of his administration I was not really all that agog over it. Bush was decidedly better than Kerry--an actual bush would be a better anything than John F'in Kerry would--but that doesn't mean that Bush was a right-winger.
2008, McCain--slightly less left-wing than his opponent, Obama, who won, partly because McCain refused to fight for the job. 2012, Obama versus Obama Lite. Yeah.
And throughout that time, GOP legislators who absolutely would not fight at all for the voters who elected them. And still won't, not even emboldened by having a President who relishes acting as their lightning rod. And who frequently voted--and vote--against the wishes of their electors, letting Democrats win even when there's no reason to.
There's not a lot to be done about it from here, except to hope and pray that the GOP--the stupid party--is not that abysmally stupid.