The Republican party certainly has its problems: a chaotic presidential race, a despised congressional party, unpopularity among the rapidly growing number of non-whites.
But the Democratic party has its problems as well. One of them is the fact that the two signature policies of Barack Obama—the Affordable Care Act or Obamacare, the nuclear agreement with Iran—are unpopular with most voters.
You heard little about Obamacare or the president's approach to Iran from the president during his 2012 re-election campaign. You heard little about them from Democratic candidates for Congress in 2010, 2012 and 2014 or from Democrats running for Congress next year.