Ronald Brownstein pithily sums up our current electoral dilemma:
Republicans can't attract enough minorities to consistently capture the White House. Democrats can't win enough whites to consistently control Congress.
Neither party has a lock on any branch of government. But Republicans are getting weaker and weaker nationally, which makes it very difficult for them to capture the White House. Midterm elections, however, which feature lower turnouts and depend on state and district voting, pose a problem for Democrats.
Obviously details still count. Republicans have a good chance of taking the Senate this year because Democrats are defending a lot of weak seats. Conversely, Democrats have a good chance of taking the Senate in 2016 because Republicans will be defending a lot of weak seats. Nonetheless, we do seem to be entering an era in which Democrats have an ever stronger edge in presidential elections and Republicans have an ever stronger edge in congressional elections, especially midterms. Unless something changes, we can probably look forward to divided government for a long time.