The poll by the Pew Research Center and USA Today showed the Republicans moving ahead with 47 percent of registered voters surveyed saying they would vote for a generic GOP candidate, while 43 percent said they would pick a Democrat. In October, a Pew/USA Today poll found 49 percent planning to vote Democratic while 43 percent said they would vote Republican.
Republicans tend to have a built-in advantage in non-presidential years because older white voters are more likely to get to the polls. As a result, polls of registered voters tend to understate the party's share.
The poll found that President Obama's approval rating remains low at 44 percent. But it is still considerably higher than President George W. Bush's 32 percent rating six months before the 2006 midterms.
Almost two-thirds, 65 percent of those polled, said they hope the next president adopts different policies, instead of continuing Obama's.
Most voters said they disapprove of congressional leaders, with Republicans getting even worse ratings than Democrats.
More than half, 54 percent, of respondents said their decision on how to vote in November is not based on their feeling about Obama. But 24 percent said they would effectively be voting against him and only 16 percent for him.
The poll was conducted between April 23 and April 27 and surveyed 1,501 adults, 1,162 of them registered voters.