Once it does and when significant crimes are announced. Democrats would be unwise to not impeach regardless of what republicans would do. Democrats would look like spineless cowards if they refuse to impeach out of fear of not getting enough republican votes in the Senate. The false equivalency of comparing Trump to Clinton's situation is just that a false equivalency. Clinton's was already popular before impeachment his approval rating was over 50% while Trump's approval rating is in the 30's. Clinton was being impeached for trivial reasons compared to the crimes of Trump. Clinton was in his 2nd term and could not be reelected anyway. The only President to be impeached in his first term was Andrew Johnson and not only did he not get reelected but he lost in a primary.
I firmly believe if Trump is found to have committed crimes Democrats should impeach. Not only is it the right thing to do, but it also makes good political sense. Impeach him and force Senate republicans to either do the right thing and vote to remove him or vote to defend a known criminal. If they chose the latter Democrats should beat them over the head with that vote without mercy.
Even if Trump isn't removed from office he will probably lose pretty badly in 2020 that is if repukes don't run a primary against him before then.
We should not fear the base of a president who lost the popular vote by nearly 3 million. Both Bill Clinton and Richard Nixon were reelected in legitimate landslides Trump's base isn't close to either of them.