By my calculations (though I’m no accountant), 1/3 is not a majority.
Though he has an extremely loyal base, he has also created a great rift on the Republican side. I wouldn’t be surprised if the Party refused to nominate him as their candidate. He’s political cyanide for many in Congress.
I believe the people in the GOP who oppose Trump will end up losing their primaries to more extremist candidates in 2018. Plus, the GOP won’t be facing an uphill battle like the democrats will be in the election: there are about 20 seats at stake, and half of them are in red states. So when the new congress convenes in 2019 talks of impeachment will be dead, or at the very least removal will be impossible with 60 senators loyal to Trump.