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.
