The presidential popular vote is one thing. But the senate (due to the prevalence of more small, conservative states) and house (due to gerrymandering) are heavily tilted in the Republicans' favor.
So the Dems have a very small margin for error, and Republicans will obstruct as much as possible if they get any power. If Republicans get control of the senate, for example, that means no supreme court seats can be filled by a Democratic president.
I can't see a third party fracturing the Republicans (I doubt one would form in the first place). Conservatives are very good about coming together to vote against people who they don't like. This is in contrast to people on the left, who are much more prone to stay home if they don't like a candidate.