I don't what the Democrats are going to do, but their failure to come out of the box running in January, or at least the second the Mueller report dropped, due to I think general lack organization and laziness in the first instance and cable news brain in the second (or, simply, we thought Mueller would save us but he didn't convince the NYT and CNN so shruggy), has made impeachment the only sensible option. If they'd decided to stand up to the bullying - essentially saying 'cut this shit out' by actually demanding people obey subpoenas and answer questions - they probably could have forced them to behave enough that the "delay until election season" strategy would work.
The answer to "what're you going to do about it?" was, too many times, "absolutely nothing."