If there's any group you want on your side when approaching an election, it's enthusiastic independent voters. Independent voters carry elections in swing districts, and (even in a poll of likely voters) independents that feel strongly are presumably more likely to vote and less likely to waver.
In the average of the last two Rasmussen polls that asked which party voters would support on a generic congressional ballot (by a margin of 7.5 percentage points, they'd favor the GOP), voters say that they disapprove, rather than approve, of President Obama by a tally of 53 to 46 percent. From Obama's perspective, that's bad enough. What's worse is that 47 percent of independents strongly disapprove of him, while only 17 percent strongly approve. Round up a half-dozen independent voters and, on average, three will strongly disapprove of the president, one will strongly approve of him, and two won't feel strongly either way. That's not a good sign for the president's party heading into a midterm election.
Independents backed Obama en masse two years ago. So, what has changed? In showing long-term trends, Rasmussen doesn't break down issues by party allegiance, but among all likely voters, the biggest swing in voters' trust in the respective parties has been on the issue of health care, on which voters' faith has swung 21 points toward the GOP since the time of Obama's election—compared to 17 points on the economy and 16 points on both immigration and taxes.
When you consider that Obamacare not only impacts health care but also the economy and taxes, and that it's both the quintessential example and the principal symbol of federal overreaching and overspending, the answer to "What changed?" would seem to be that the Democrats rammed Obamacare down the American people's throats and are now poised to pay the price. This has long been clear, and voters have plainly indicated this to pollsters when given the chance.