They could have chosen to censure him - which wouldn't have to go to the Senate.Democrats are in a bad place, and it's not by their doing. They are damned if they do, and damned if they don't. Obviously, choosing to impeach with a GOP-led Senate is not going to end well, and will make Trump think he's been exonerated, and make the Democrats look bad. However, had they have chosen not to impeach, it would have shown the world that the president is above the law, and would have further emboldened Trump to do whatever he wants. So there's really no way to win here. This is the Constitutional crisis Trump, McConnell, and the GOP at large have created.