Including independents who describe themselves as leaning toward a party, Democrats have a 13-point advantage over Republicans. Among registered voters, 51 percent identify with the Democrats while 38 percent describe themselves as Republican, the Pew report said.

In 2004, 47 percent identified with Democrats and 44 percent with Republicans.

The Democrats have made substantial gains among two groups of voters, the report said. The party has a 20-point lead among young voters, up from 10 percent in 2004, and a 14-point lead among moderate income voters, a group that tilted Republican four years ago.

Voters viewed the Democrats far more favorably, with 57 percent liking the party and 37 percent disliking it. The Republican Party did less well with 43 percent having a favorable view and 49 percent an unfavorable one.