Ummm... they just nuked that trend despite how gerrymandered this country is to hand it over to the Republican party. There is no 'cusp' about it. Republicans turned on their own and showed up for Democrats and implementation of Checks and Balances.
The Dem's lost 1.000+ elected seat during Obama's 8 years. In 2010 mid-terms Obama lost 63 seats in the House and 6 seats in the Senate !
It's almost as if rigging the vote in safe districts doesn't help nation wide...
It is called 'gerrymandering'. Because of our corrupted election system, Democrats have to turn out in far greater numbers to break even.
Gerrymandering