If the Democrats lost the presidency, it’s a near certainty that the party wouldn’t regain control of the Senate or the House, where Republicans already enjoy comfortable margins. Republicans could control in excess of half the governorships and state legislatures in 2017.
And Republican appointees are a majority on the Supreme Court, which after some liberal-leaning decisions on the Affordable Care Act and gay marriage, will likely tilt conservative again.
Such a scenario would make Republicans even more powerful than when they held all the levers of power from 2003 until 2007: Then, they didn’t control as much on the state level, and in Congress there still were moderate to liberal party lawmakers, such as Arlen Specter and Lincoln Chafee, who often voted with Democrats. (Both subsequently became Democrats.)
The Democrats, who consider themselves the governing party, would be in their weakest position since the 1920s.