Meh. So long as he has title of President he's largely untouchable. Even if the dems manage to take both sides of congress, the craven GOP will never convict on charges for impeachment.
I wasn't strictly talking impeachment but let's consider likely consequences to midterms. At the very least the House is Dem, but there's also an apparent very real chance to take the Senate.
In the case of just the House, the Dems control legislation coming out especially for those regarding Federal monies. If the Senate is gotten too then the Republicans are powerless and sidelined. The Dems will start disassembling the entire Republican agenda and more, implement their own to compensate. Also the Dems can have as many committees and investigations into Trump, his family and anyone including Republican Congressmen as they please. The Dems can destroy the Republican with extreme and justified prejudice and there's nothing they can do.
They could gut and eat the GOP with enthusiastic support from a great number of people, or cut their losses and go with Pence. They'll still have their programs dismantled but they won't automatically be subject to investigations involving possible illegal activities, something entirely possible.
So the question after midterms is who is immolated, Trump or the Republicans themselves. Will they accept huge tax increases on their friends and supporters and have their legislation held hostage?
How many limbs are Republicans willing to saw off the elephant to save Trump?