U.S. Senate candidate Matt Bevin talks with members of the Roatary Club in Ashland, Ky. Monday Jan. 6, 2014 where he was a guest. (AP Photo/The Independent, Kevin Goldy) (Kevin Goldy/AP)

The jabs at Mitch McConnell have been coming on the air and in the mail, accusing the Senate Republican leader of helping President Obama and letting down Kentucky voters.

The attacks are not the work of McConnell’s conservative primary challenger. Instead, they are coming from independent groups that want to unseat him and are benefitting from a new system of unlimited contributions ushered in by the Supreme Court in its 2010 Citizens United v. FEC ruling.

Conservatives cheered that decision, described at the time by McConnell as a “First Amendment triumph.” But, as McConnell’s experience now illustrates, the loosened rules are causing political headaches for the GOP — allowing tea party groups to bring more financial firepower to challenges against Republican incumbents.

Now, party leaders struggling to unify the warring wings in time to retake the Senate this year and lay a foundation for the 2016 presidential race are instead contending with a well-financed insurgency.

Republicans are now far more likely than Democrats to field attacks by independent groups in their primaries. In 2012, super PACs and nonprofit groups reported spending nearly $36 million in GOP congressional primaries, compared with less than $10 million in congressional Democratic primaries, according to a Washington Post analysis of campaign finance records.

A similar dynamic played out in the 2012 Republican presidential primaries, when the contenders were pummeled by super PACs aligned with their rivals. This year, the attacks by the GOP’s tea party flank are spurring a financial arms race, as major ­center-right groups and business organizations step forward to bolster incumbents — an indication that the 2014 primary battles could be bloodier than past cycles.

“Right now, the Republicans all have their cannons aimed at each other,” said Katon Dawson, a former South Carolina GOP chairman who is heading a new group, the West Main Street Values PAC, to help Sen. Lindsey O. Graham (S.C.) fend off challenges from four primary opponents, “and the Democrats are getting a free ride.”

The 2014 battle lines are being drawn by powerhouses such as the Club for Growth, which launched a “Primary My Congressman” effort last year to take on centrist Republicans. The organization is already backing challengers to eight-term Rep. Mike Simpson (Idaho) and Sen. Thad Cochran (Miss.) and may engage in more primaries.

The Senate Conservatives Fund, a group started by former senator Jim DeMint that promoted last year’s government shutdown, has rallied behind conservatives taking on McConnell, Cochran and Sen. Pat Roberts (Kan.).

In Kentucky, the organization has already devoted nearly $1 million to help Matt Bevin, the conservative candidate challenging McConnell in the primary. More than half of that amount has gone to ads and mailers. Abruising TV spot it ran in the state in November charged that McConnell “helped Barack Obama and Harry Reid fund Obamacare.” The group has also given a large share of its funds directly to Bevin’s campaign.

“We believe that primaries are good, and voters deserve to have choices,” said Matt Hoskins, executive director of the Senate Conservatives Fund. “We are going to do everything we can to help Matt Bevin win this race.”

In January, the Washington-based tea party organization FreedomWorks announced its endorsement of Bevin, saying it planned to spend at least $500,000 on get-out-the-vote efforts on his behalf. Matt Kibbe, the group’s president, said it will jump into more races in the coming weeks.

Conservative activists say they are making the party stronger, helping elect figures such as Sen. Ted Cruz (Tex.) and Sen. Marco Rubio (Fla.).

“If you look at our track record, we’ve successfully repopulated the Republican Party with some pretty compelling young leaders,” Kibbe said.

The fierce infighting has not prompted a widespread reassessment among Republicans about the merits of loosening campaign finance rules.

Jesse Benton, McConnell’s campaign manager, said the senator is an “unapologetic champion of the First Amendment,” and “if that means that a byproduct is that he has to take a little extra heat, he’s going to do it.”

Still, Benton acknowledged, the outside attacks are “a distraction and a nuisance that we would just as soon do without.”

In several key ­races, the tea party groups have not yet wielded the resources of Republican incumbents and their backers. But the groups have succeeded in forcing incumbents to engage early. By the end of September, McConnell had already dumped nearly $4 million into what is expected to be one of the most expensive Senate races of the cycle. McConnell also faces a strong Democratic challenger, Alison Lundergan Grimes, Kentucky’s secretary of state.

The escalating tensions alarm many in the party, who say tea party groups are diverting time and money that could be better spent.

“Our best advocates and resource for retaking the Senate are Republican senators, and there’s an opportunity cost of mindlessly primarying them for primary and profit sake,” said Brad Dayspring, a spokesman for the National Republican Senatorial Committee.

Centrist GOP groups and industry organizations such as the U.S. Chamber of Commerce are firing back, laying plans to spend heavily to bolster McConnell and other establishment Republicans. They worry about the losses of tea party candidates in past cycles and the political damage experienced by the party during last year’s government shutdown.

“It’s important there’s a counter­balance to groups looking to profit from candidates who are either unelectable in a general election or unwilling to govern in a conservative fashion in Congress,” said Dan Conston, a spokesman for the nonprofit advocacy group American Action Network, which is chaired by former senator Norm Coleman (R-Minn.).

Main Street Partnership, a group led by former congressman Steven C. LaTourette (Ohio), plans to raise $8 million through a super PAC and nonprofit organization to help centrist Republicans such as Simpson, the Idaho congressman.

Donors have been enthusiastic, said Sarah Chamberlain, the group’s chief operating officer. “For the first time, they’re starting to understand that they’re at war, and they have to step up,” she said.

The expanding role of independent groups in campaigns has accelerated the calls by McConnell and other GOP leaders to lift the limits that remain on contributions to the now-weakened national party committees. Individuals cannot give them more than $32,400 a year.

The Supreme Court is considering a case, McCutcheon v. FEC, that could begin to chip away at those caps.

“Because of fundraising restrictions on the political parties, their share of the independent expenditure pie has shrunk dramatically,” said Michael Toner, a Republican campaign finance lawyer. “Outside groups are playing more and more of a role, and that’s empowering the far right.”

Many of the conservative groups active in elections this cycle predated Citizens United, but they relied largely on traditional political action committees, which can only accept donations of up to $5,000.

In the Citizens United case, the Supreme Court said that corporations could spend unlimited sums on political activity. That decision was cited a few months later by a lower court, which said there could be no restriction on the size of donations to independent political committees — a ruling that paved the way for super PACs.

Conservative groups were among those that embraced the new vehicles. Club for Growth, which spent $10.3 million in the 2008 cycle, poured $23.4 million into races in 2012 — including nearly $17 million through its super PAC Club for Growth Action.

More than $12 million of the money that Club for Growth Action raised in the last cycle came in the form of six- and seven-figure checks from donors such as tech investor Peter Thiel, private equity titan John Childs, New Jersey investor Virginia James and Texas home builder Bob Perry, who each gave at least $1 million, according to campaign finance records.

Perry, who died in April, also gave a $1 million donation last year to the super PAC arm of Senate Conservatives Fund, a sizable chunk of the $9 million the group said it raised in 2013.

Conservative activists said that while they get big checks, one of the most important roles they play is in bundling small donations that go directly to the campaigns of the candidates they are backing.

“The average amount that our donors give to our candidates is just $37,” said Hoskins, of the Senate Conservatives Fund. “Super PACs can certainly raise large amounts from a few donors, but nothing is more powerful than having 10,000 people invest in a candidate.”

Alice Crites contributed to this report.