Here’s a scenario. Donald Trump stonewalls on talking to special counsel Robert Mueller. He decides he has nothing to gain by answering his questions. Mueller cannot subpoena the President, so we are done.
Or are we?
Assuming the Dems take power in the House (not a safe assumption at all), a number of House committees will start investigating Trump for this and that. He will stonewall all of them. And there the matter will end.
Or will it?
Suppose the House votes to impeach Trump, knowing that they have no hope of winning a conviction in the Senate. But the reason for impeaching him is not to kick him out of office. It is to force a trial in the Senate where the President will have to testify under oath. In other words, the House tells Mueller, “Give us your questions – we’ll get the answers for you”.
Mitch McConnell will do everything he can to delay a trial. Brett Kavanaugh will likely help him. But you have to admin it would be a pretty sneaky way for the Dems to get Mueller the information he needs to build his case(s).