Because senate bills need 60 votes to pass and there are just 54(?) republican senators right now. Senate "being the key" probably referred to impeachment-removal protection and presidential appointments. Several years back the senate democrats changed the rules for approving presidential appointees from 60 votes down to a simple 51, so their golden boy could get people he wanted. Now they can only sit there and watch (helplessly) as Trump's appointees sail through. The democrats were probably thinking they wouldn't have to worry about that rule change coming back to bite them in the ass, because they were going to arrange guaranteed victories at every level from crooked onward.