Right now, Congress could impeach on the charges of obstruction of justice (in the case of the Comey firing) and defiance of the Emoluments Clause (which is blatant).
However, impeachment on these grounds of a Republican President by a Republican Congress is unlikely. If it was a Democratic President, such as Hillary Clinton, who had committed these offenses, it would be a different story.
When the Mueller and FBI investigations are completed and and if they show collusion with the Russians on Trump’s part to influence the election, that will probably do it. We know that wrongdoing was done on our side, by both Junior and Manafort. The most exoneration Trump might get with these investigations is that he didn’t know what was going on (and even that’s a stretch). And while that might save him from impeachment, it’s going to hurt Republicans far worse than it will hurt Democrats.