The Democrats are faced with an unenviable decision. The Mueller report justifies impeachment. Congress has a constitutional responsibility to impeach.
On the other hand, any attempt at impeachment is doomed to failure. Impeachment needs to be a bipartisan act, and the current polarised state of US politics makes a successful impeachment inconceivable, whatever Trump has done.
The Democrats are well aware that the public at large is not going to like what will be seen as a political decision to impeach. It will hurt them at the ballot box, and might well assist Trump in getting reelected.
Strictly according to their oaths of office, Congress ought to impeach. While this may seem cowardly, I think they can and should take the position that the good of the country means they must desist, concentrating on beating Trump at the polls in 2020.