The Constitution provides that the President can be impeached by the House for “high crimes and misdemeanors” (upon impeachment he/she is then tried by the Senate, with the maximum penalty removal from office with a lifetime ban on public service). The Constitution is silent on when the crimes need to have been committed. As is so often the case in these Constitutional gray areas, the framers intended (as evidenced by their notes) that the members of the Legislative branch use their collective judgment to decide these things, and not to constrain future Constitutional actors with too many rules. So the House could impeach Trump for events during the campaign if they felt such events disqualified him from being President. The reality is that the House won’t contemplate impeachment until the public (including a large number of Republican voters) demands it.
What would sink Trump is if it were ever proven that members of his campaign colluded with a foreign government to commit felonies in the pursuit of partisan advantage, and Trump knew about it. That makes Trump himself a co-conspirator in the commission of a felony – the kind of “high crimes and misdemeanors” that the Framers considered to be evidence of the sort of moral turpitude that should be disqualifying for “any office of profit or trust under the United States or any of them”.