In this cycle’s costliest and most competitive House and Senate races, Members of Congress are deploying a little-noticed but influential weapon: their personal political action committees, typically known as leadership PACs.

Long a staple of Member-to-Member campaign giving, leadership PACs are now so ubiquitous that more than three-quarters of all lawmakers in Congress have them, and their influence is growing. Once criticized as a means of skirting campaign contribution limits, leadership PACs are now testing the more controversial waters of unrestricted money.

Some Member-run PACs have started endorsing candidates or bundling money for them, operating as quasi-political-party committees. Others have funneled large sums to unrestricted super PACs. One leadership PAC, previously operated by Sen. Jim DeMint (R-S.C.), even morphed into a super PAC, signaling what some argue is an inevitable trend.

“Soon, Members of Congress, I think, will all have their own super PAC,” said Sheila Krumholz, executive director of the Center for Responsive Politics.

Though some top Congressional leaders are cozy with, and have raised money for, super PACs, most have stuck with the more familiar vehicle of the leadership PAC to help colleagues and candidates.

But the face of leadership PACs, which unlike unrestricted super PACs may still raise and spend no more than $5,000 per election, is changing. Once principally a means for ambitious lawmakers angling for leadership posts to curry favor with their colleagues, some of the PACs are becoming multimillion-dollar operations unafraid to antagonize party leaders and weigh in on primaries.

DeMint set the pace when he launched the Senate Conservatives Fund in 2008, pioneering a new model built on vetting and promoting like-minded conservatives and rounding up or “bundling” small checks for them. The PAC, which raised $9.3 million in the 2010 cycle, became so influential that it spun itself off as a super PAC in July. DeMint now has no official ties with the group.

“It has definitely boosted our fundraising and helped us do some things in this cycle that we would not have been able to do,” Executive Director Matt Hoskins said of the PAC’s transformation. The PAC has two prongs: the Senate Conservatives Action super PAC and the conventional PAC, dubbed the Senate Conservatives Fund. The group has raised $12.8 million this cycle.

At least two GOP leadership PACs — the Reclaim America PAC, operated by Sen. Marco Rubio (Fla.), and the Constitutional Conservatives Fund, operated by Sen. Mike Lee (Utah) — have emulated DeMint. Reclaim America has helped four GOP Senate candidates with $100,000 this election cycle, PAC Director Terry Sullivan said, both in direct contributions and donations bundled on their behalf.

“I think this is a new avenue [leadership PACs] can explore, and I think more probably will,” Sullivan said.

Lee’s leadership PAC also vets and endorses candidates who share Lee’s tea party philosophy of limited government. Lee sought Federal Election Commission permission to set up an unrestricted super PAC in December, but commissioners unanimously turned him down.

Rep. Ron Paul (Texas), who gained a national grass-roots following during his  unsuccessful GOP presidential primary bid, is also using his leadership PAC, Liberty PAC, to promote a half-dozen Senate and close to two dozen House GOP candidates.

“We absolutely follow Sen. DeMint’s model,” said Lee’s communications director, Michael Phillips, who said the PAC has a long-term fundraising target of $5 million to $10 million and hopes to coordinate in the future with like-minded PACs such as the Senate Conservatives Fund and Liberty PAC.

Democrats have leveraged their fundraising prowess in other ways. The Searchlight Leadership Fund, run by Senate Majority Leader Harry Reid (Nev.), has directly donated $5,000 and $10,000 checks to dozens of Democrats, but it also gave $255,000 to Majority PAC, the leading super PAC backing Democratic Senate candidates.

Reid’s leadership PAC donated the maximum $10,000 to Sen. Claire McCaskill (D-Mo.). But Majority PAC has made close to $870,000 in campaign expenditures on McCaskill’s behalf.

Similarly, the leadership PAC run by Sen. Barbara Boxer (D-Calif.), PAC for a Change, also has given $205,000 to the Majority PAC.

“They’re leveraging their special reputation and cachet with donors to spread the wealth in very targeted ways,” Krumholz said. The CRP estimates that close to 400 Member-run leadership PACs have given out more than $32 million this election cycle.

As they’ve gained leverage, some leadership PACs have drawn controversy.

House Majority Leader Eric Cantor (R-Va.) ruffled GOP feathers earlier this year when his leadership PAC, Every Republican is Crucial PAC, gave $25,000 to a super PAC known as the Campaign for Primary Accountability, which was busy trying to oust some of his colleagues.

The Missouri Democratic Party filed complaints with the FEC and the House Ethics Committee last month after DeMint endorsed GOP Rep. Todd Akin in the Missouri Senate race. Democrats said Akin had improperly negotiated and coordinated with the super PAC.

Hoskins said the super PAC operates independently from campaigns and endorsed Akin only after surveying its members. He said DeMint does no fundraising for the super PAC, which has taken pains to sever ties with the Senator.

Still, watchdogs who fought hard to improve leadership PAC disclosure with ethics reforms enacted in 2007 bemoan the groups’ growing clout.

“We finally got registration and disclosure of them in 2007, and I thought that was a big step forward,” said Government Affairs lobbyist Craig Holman of Public Citizen. “And now the leadership PACs are doing exactly what they were not supposed to be doing, and that is electioneering.”