As it stands now, it seems like James Comey wasn't fired due to any cover-up over Russia, but rather because Trump was annoyed that Comey has been unwilling to become one of his Yes-men.
And somehow, Trump deluded himself into thinking that it would work because Comey was "unpopular with Democrats". He's got the political instinct (and the temperament) of a five-year old.
It doesn't matter what the Democrats think. The GOP Senate has already circled its wagons around Trump, giving him cover whatever the motive of the firing.