I think the argument should've been that Trump isn't that different from many politicians in a key way (promising things he can't deliver). Not sure how well that would've worked if Clinton was the one making the argument, but it's clear that arguing Trump was completely different from any other politician helped him enormously.
Hillary's courtship of republicans was against the advice of the DNC which had warned her not to do it. The DNC told her campaign that it was hurting Democratic downballot performance.
https://newrepublic.com/minutes/137093/clinton-campaign-decision-made-may-doom-down-ballot-democrats
Now of course, the leaks themselves are another factor, with Putin slamming the democrats, and praising Trump, would he have even released the emails to help a generic republican candidate even more anti-russian than Hillary? Of course not. So, no DNC leaks, which would've helped dems with their turnout.
The Clintons are selfish and don't actually care about electing down ticket Democrats. Before Bill won, the House had been Democratic for 38 years.
This is somewhat unfair to Bill Clinton, and even Hillary; they have done much for downballot Democrats over the years. If they had not done so, they would not have had the kind of structural advantages over Sanders Hillary enjoyed in the primaries. The going in the tank for Hillary by the DNC was a pretty big "Thank you!" for years of service.
The house shifted to the GOP in 1994 in no small part due to reapportionment in the South, where blacks were consolidated into "black districts" and white Democrats were unable to prevail without the infusion of black support that had previously put them over the top. Liberals were all for this when they thought the South would continue to elect Democrats to Congress, but when the end of the process was a dominant GOP and a Democratic Party that was mostly black, they weren't so fond of the idea.
