Over the summer, as conservative energy came to its anti-tax, anti-spending, anti-stimulus crescendo amid a wave of town hall and tea party-style protests, Republicans started overtaking Democrats in generic House balloting--polls that ask people, regardless of the candidates they may get to choose from, which party they're likely to vote for in House of Representatives races in the 2010 midterms.
No less than 10 polls in August and September had Republicans leading, and one Rasmussen survey had Republicans lup by 7 percentage points. Lots of polls had Democrats ahead, too, and the average, according to Pollster.com, never quite swung into Republicans' favor--but, for a moment in August, it was very close.
Now things appear to have swung back into Democrats' favor, at least slightly. Rasmussen (which is typically more friendly to the GOP) still has Republicans ahead, but polls released in the past few days by Ipsos/McClatchy and CNN have Democrats leading 48-41 and 51-41, respectively, though, when CNN restricted its sample to registered voters, Dems were only ahead 50-44.
See Pollster.com's track of the progression (which does not yet include the Ipsos/McClatchy poll) here:
Conventional wisdom says Republicans will take a few seats back in 2010. There's a natural ebb of Democratic success after that they took control of Congress in 2006 and the White House in 2008, an eventual 60-seat majority in the Senate. They're also defending more seats than Republicans (CQ recently rated the top ten most vulnerable House seats, and nine of them are held by Democrats).