In the shadow of the super-size super PACs that have reshaped the battle for the White House and Senate, a new and potentially potent kind of super PAC is rapidly proliferating in the closing weeks of the campaign and taking aim at House races.

With some of the groups backing Democrats and some supporting Republicans, they are picking a few congressional races in which advertising is cheaper or the airwaves less cluttered — and transforming them with a concentrated barrage of outside money, swamping incumbents and challengers alike.

In Utah and Georgia, a group known as Center Forward, headed by a retired Democratic lawmaker turned Beltway lobbyist, has spent $1 million attacking two Republican candidates. In Florida, Treasure Coast Jobs Coalition has spent nearly $1 million against Patrick Murphy, a Democratic candidate, and supporting Rep. Allen B. West, the Republican incumbent.

Now or Never PAC, a Missouri-based group, has spent more than $900,000 to aid a Republican incumbent in neighboring Illinois, Rep. Joe Walsh, a Chicago-area lawmaker who had been outspent by his Democratic challenger until the group entered the race.

"It doesn’t make any sense for us to spend the several million dollars that we expect to raise in the Connecticut Senate race," said Tyler Harber, a strategist for Now or Never. "We’ve focused on races where our involvement could have a positive impact — and perhaps bring other groups in."

The emergence of smaller super PACs helped fuel a surge in September advertising by outside groups in House races: Far more money was being spent far earlier than for the 2010 elections, when Republicans won control of the House.

Through the beginning of October, super PACs and other outside groups reported at least $38.5 million in independent spending to the Federal Election Commission, nearly seven times as much as they spent during the same period in 2010. Almost all of that money was spent in September, according to a analysis, as outside groups reacted to swings in the presidential race and exploited redistricting, which has left many incumbents with thousands of new constituents who are unfamiliar with them.

And Democratic-leaning groups have narrowed the spending gap, with about $19.8 million of the total backing Republicans and $18 million backing Democrats. Those totals do not include issue ads that groups are not required to disclose to the FEC or the significant spending — of both disclosed and secret money — that big-spending groups like the U.S. Chamber of Commerce have already planned.

The large disparity in outside spending that helped Republicans win the House in 2010 did not emerge until after September, and a similar surge by major outside groups is likely in the coming weeks. But the early spending this year suggests how much impact smaller outside groups believe they can have in House races in which the candidates themselves spend far less money and the cost of influencing voters can be lower than in the presidential or Senate contests.

The donors and political consultants behind many of the groups only began officially incorporating them over the summer, meaning they have not yet had to file any disclosures that would reveal ties between the interests financing the attacks and the lawmakers benefiting from them.

But other groups that have filed disclosures appear to be mimicking some of the big super PACs that played a pivotal role in the Republican presidential primaries this year, serving as vehicles for wealthy donors, friends and family members to pour additional money into a race after they have already given the maximum allowed to the candidate’s own campaign.

American Sunrise, which is supporting Murphy in the Florida race, has raised $250,000, most of its reported funding, from Murphy’s father. A few donors have also provided much of the money for Now or Never PAC: Of the $484,000 the group has reported raising, $250,000 came from Stanley Herzog, a Missouri construction magnate.

In September, Center Forward, founded by former Democratic Rep. Robert E. Cramer of Alabama, spent $994,840 on ads opposing Mia Love and Lee Anderson. The two Republicans are running against Reps. Jim Matheson in Utah and John Barrow in Georgia, who were members with Cramer in the Blue Dog Coalition, a caucus of socially conservative Democrats.

In New York’s 1st District, Prosperity First Inc. has spent close to $500,000 on advertising to support Randy Altschuler, a Long Island businessman and Republican who is running for a second time against Rep. Timothy H. Bishop, a Democrat.

Most of the group’s money has come from Robert Mercer, a top executive at the hedge fund Renaissance Technologies and donor to Altschuler, with whom he shares a fierce distaste for the Dodd-Frank financial regulations. Mercer has also given $1 million to American Crossroads, which recently spent another $277,363 on the race through an affiliated group. (The founder of Renaissance, James Simons, is the country’s leading donor to Democratic super PACs.)

Mercer declined to comment, as did a Washington-based election lawyer listed on filings as the group’s treasurer. But Altschuler’s campaign said Prosperity First had helped transform the race, which one mid-September poll showed Bishop leading by double digits.

"They’ve made it more competitive," said Diana Weir, Altschuler’s campaign manager. "And I think as the race gets more competitive and they see a tighter race, everyone’s going to want to come in."

Unlike the largest groups, which operate with all the trappings of a presidential campaign — websites, spokesmen, news releases announcing major advertising buys — the smaller groups often fly under the radar, rarely announcing themselves to the world beyond their ads. In some cases, little is known about the groups save for the name of the person filing the incorporation papers.

And while the major groups intervene in dozens of races and function as auxiliaries of each party, the smaller groups often pursue narrower agendas or are linked to specific candidates.

In California, for example, a San Francisco-based group called America Shining has focused on one race, spending $150,000 to help Jay Chen, a Democrat, in a Republican-dominated district south of Los Angeles.

American Unity PAC, another new group, was founded by Republicans who support gay marriage and is financed in part by some of the same big donors, such as Paul Singer, a wealthy hedge fund investor, who have poured money into larger groups. American Unity spent $517,850 in September to oppose Bill Foster, an Illinois Democrat challenging Rep. Judy Biggert, a pro-gay-marriage Republican.

Friends of Democracy, a group financed in part by Jonathan Soros, the son of billionaire financier George Soros, is dedicated to campaign finance activism. The group’s co-founder is David Donnelly, the executive director of Public Campaign Action Fund, which supports tighter campaign regulation, including overturning the Supreme Court’s Citizens United decision, which paved the way for super PACs.

Since August, the group has spent $722,948 on ads opposing Republicans, much of it in races in New Hampshire, California, Wisconsin and Minnesota. Donnelly said that the group was looking for races in which relatively few outside groups were active and it could help candidates who would take a strong stand on tighter campaign regulation.

"We don’t have the kind of budget to go in with million-dollar TV buys," Donnelly said. "We think that’s a blunt instrument, where we can go in with a scalpel."

Donnelly said that he was not blind to the irony of running a super PAC dedicated, in essence, to abolishing super PACs.

"I am hoping to work myself out of a job," he said.