So far, Democrats under Obama are down five Senate seats (from 51 in 2006 to a likely 46 today, counting independents who caucus with them) and around 45 House seats, depending on final recounts. Three times since World War II, a two-term president's party has lost more combined congressional seats by the end of the race to succeed him: During Clinton's presidency, Democrats lost six Senate and 56 House seats; under George W. Bush, Republicans lost 14 Senate and 45 House seats; behind Dwight Eisenhower, the GOP lost 12 Senate and 46 House seats.
The other post-World War II presidents performed better than Obama. (The numbers were 46 combined Senate and House losses under John Kennedy and Lyndon Johnson; 42 under Richard Nixon and Gerald Ford; and 41 during Harry Truman's truncated two terms.) Only Ronald Reagan defied the pattern: Behind him, Republicans gained four Senate and 17 House seats.
The 2016 election will fix Obama's final place on that list—and determine whether his party will hold the presidency when he departs. The only two-term president since World War II who passed that test was, again, Reagan, whose vice president, George H.W. Bush, succeeded him in 1988. Looking back further, the pattern seems to be that a popular outgoing president can't guarantee that his party will succeed him (it worked for Reagan, and for Theodore Roosevelt in 1908, but not for Eisenhower or Clinton); but a deeply unpopular outgoing president almost always ensures his party's defeat (Truman, Johnson, George W. Bush, and Woodrow Wilson in 1920).
That shouldn't be surprising. When voters want change, it's natural for them to look toward the out party to deliver it. That instinct makes disenchantment with Obama the greatest potential headwind facing Democrats in 2016.
A big lesson from 2014 is that Democrats can't tame that gale by ignoring it. Even Democrats who shunned Obama this year found no shelter. Voters who disapproved of Obama gave Republicans at least three-fourths of their vote in 18 of the 22 Senate races for which exit polls were conducted. The voter blocs most hostile to Obama stampeded to Republicans everywhere. No Democratic Senate candidate in a competitive race carried white men or whites without a college degree; in those races, only New Hampshire's Jeanne Shaheen and Michigan's Gary Peters tied or won among whites overall. The receding tide lowered all boats.
If Hillary Clinton runs in 2016, she will bring her own well-burnished brand and relationship with voters to the race. But it's delusional to imagine she would be immune to attitudes about Obama: Exit polls in 1988 and 2000 found that 88 percent of voters who disapproved of Reagan and Bill Clinton, respectively, voted for the other party's nominee to succeed them. (In 2008, Obama won two-thirds of Bush disapprovers.) No matter how she positions herself, Clinton will always represent more continuity with Obama than any Republican does.