Among those targeted by outside groups: Orlando area Rep. Daniel Webster, R-Fla., the focus of $1.1 million in ads being run by Independence USA PAC, a political action committee run by billionaire New York City Mayor Michael Bloomberg, an independent.
"It's Michael Bloomberg coming in from storm-ravaged New York. I don't think in our district that if that were known that would play very well," said Webster campaign consultant Kirsten Borman.
The American Action Network, affiliated with House GOP leaders, launched an ad attacking Webster's Democratic rival, former Orlando police chief Val Demings, for supporting Obama's health care revamping.
Mitt Romney recorded a TV spot for Republican House hopeful Mia Love of Utah, where the GOP presidential candidate is overwhelmingly popular. "I trust Mia to protect seniors," Romney says.
Those in uncertain races include about two dozen House GOP freshmen from the 87-strong class of newcomers whose rise to power was energized by tea party voters. Among them is Rep. Scott DesJarlais from central Tennessee, a physician and abortion foe whose re-election seemed assured until recent reports that he once pressed a mistress to have an abortion.
The nation's most expensive competitive House campaign: freshman GOP Rep. Allen West's re-election battle in South Florida against Democrat Patrick Murphy, which has seen $17.2 million in spending. West, a firebrand conservative and prolific fundraiser, has spent $13.8 million of the total.
Republican pickups of three seats are possible in New York and likely in North Carolina, where three veteran Democrats are retiring following redistricting, further trimming the roster of both parties' moderates in Congress.
The GOP is also hoping to knock off Democratic incumbents in California and win other seats in Utah, Arkansas and Oklahoma. They could even defeat Massachusetts Democrat John Tierney, beset by a gambling scandal that has involved his wife and two brothers-in-law. Massachusetts hasn't had a House GOP member since 1997.
Democrats are hoping for big days in California and Illinois and their own pickups in New York — states where Obama is expected to win decisively. Especially in New York and Illinois, redistricting has not helped tea party-backed Republicans like Illinois Rep. Joe Walsh and New York Rep. Ann Marie Buerkle.
Reflecting areas in play, Democratic and GOP political groups have poured money in recent days into House races in Arizona, Minnesota and Rhode Island in addition to California, Illinois and New York.
Contests that pit two members of Congress against each other are often among the most bitter and costly. This year is no exception.
In Los Angeles, a pair of veteran Democrats — Reps. Howard Berman and Brad Sherman — argued nose-to-nose at a debate last month until they were separated by a deputy sheriff. Including each man's campaign and outside spending, their race has cost $13 million, according to the Sunlight Foundation, a bipartisan group that follows campaign spending.
A contest in northeast Ohio pitting GOP Rep. Jim Renacci against Democrat Betty Sutton has drawn $9.7 million in outside spending by the Congressional Leadership Fund controlled by House GOP leaders, House Democrats' campaign organ and numerous other groups.
The only House race where there has been more outside spending is the Pennsylvania contest between Democratic Rep. Mark Critz and GOP attorney Keith Rothfus, where it has surpassed $9.9 million.