Michael Barone explains:
In 2008, Obama won 53 percent of the vote, the highest percentage for any Democratic nominee in history except Andrew Jackson, Franklin Roosevelt, and Lyndon Johnson. Now he’s averaging 46 percent in recent polls.
That’s much closer to the 45 percent that Democratic candidates won in elections to the House in 2010. And in the last three presidential elections, the winning candidate has won the same percentage (or within 1 percent) as his party’s percentage in House elections two years before.
Obama had a popular-vote margin of 7 percent in 2008. But Republicans had a margin of 7 percent in the popular vote for the House in 2010. If you tote up the electoral votes in the states they carried, you find them with a 351–184 edge over Democrats (the remaining three in the District of Columbia are obviously Democratic).