For an impeachment to result in removal from office, you only need 1/3 of the House, but 2/3 of the Senate. Because only about 1/3 of the Senate is up for re-election in any given year, it's pretty much impossible for Democrats to get there in one election. So, even if Democrats take control of the Senate, they would need 15 or so Republicans to go along with it for it to stick. That is extremely unlikely.

Overriding a Presidential veto takes 2/3 of both houses. But I think that is much more likely to happen if Democrats take control of both houses, because they can start negotiating with Republicans and put enough things for them to vote for in bills to reach that threshold. Members will have an easier time voting against the President in individual bills than they would on impeachment. I think he would actually be more pissed off if multiple vetoes get overridden than he would if they successfully impeach him.