If Democrats cling to their Senate majority this fall, it will be in large part because of a well-funded group connected to Senate Majority Leader Harry M. Reid that has helped build a formidable firewall around vulnerable incumbents.

The Senate Majority PAC, fueled by billionaires and labor unions, has been the biggest-spending super PAC of the 2014 midterm contests. Together with an allied tax-exempt group, Patriot Majority, the pro-Democratic effort has poured at least $36 million into ads and voter outreach, according to the Center for Responsive Politics, a nonpartisan research organization.

The groups’ early, aggressive presence in pivotal Senate races spotlights how, four years after being dramatically outgunned in the outside-money game, Democrats are now some of its most adept players.

Led by a quartet of longtime political strategists with close ties to Reid (D-Nev.), the Senate Majority PAC has elbowed out other pro-Democratic groups and been on the leading edge of attacks against conservative donors Charles and David Koch. The group has become a fixed center of gravity in the left’s expanding constellation of super PACs and interest groups.

Perhaps most notably, the super PAC has held its own on the air against Americans for Prosperity, a conservative advocacy group that is the primary political organ of a network backed by the Koch brothers and other wealthy donors on the right. By the end of the summer, the two groups had run nearly the same volume of television ads nationwide, according to Kantar Media/CMAG data analyzed by the Wesleyan Media Project.

GOP candidates are still expected to benefit from more outside money this year, much of it invested by nonprofit groups that largely do not report their spending.

But the Democratic Senate effort has drawn the grudging respect of Republican strategists, who say it has succeeded in putting their candidates on the defensive. Even Democratic operatives whose projects did not gain traction because of the Senate Majority PAC’s dominance admire its execution.

“I would say they’ve saved the day,” said Ben Chao, a strategist who tried unsuccessfully to start individual super PACs for several Senate Democrats.

Whether the Senate effort will be regarded as heroic after Election Day remains to be seen.

The strategists hope to replicate on a broad scale the hard-fought win they helped Reid secure in 2010. That year, the Senate leader beat back a strong challenge from Republican Sharron Angle with the help of robust air cover from Patriot Majority, a liberal advocacy group run by Craig Varoga, a former Reid communications director. The victory in Nevada was a rare bright spot that year for Democrats, who lost control of the House amid a blitz of outside attacks by pro-GOP groups such as American Crossroads.

“The other side had a lot more money on the air, and there was an imperative that we couldn’t let that happen again,” said Rebecca Lambe, a top Reid operative in Nevada and co-chairman of the Senate Majority PAC.

So even as Reid has lambasted the influence of wealthy conservative donors, the super PAC run by his longtime advisers has scooped up seven-digit checks from hedge fund founders Tom Steyer and James Simons and other billionaires. Unions representing carpenters, teachers, public employees and other workers have given at least $6.6 million, according to data compiled by the Center for Responsive Politics.

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Some of the financial backing behind the effort remains hidden. As a tax-exempt “social welfare” organization, Patriot Majority — which is running TV ads and an under-the-radar field program — is not required to disclose its contributors. The group’s leaders reject accusations of hypocrisy, arguing that Democrats, unlike Republicans, are seeking to rein in the role of wealthy donors in politics.

Until that happens, however, “we can’t go forward with our hands tied behind our backs,” said Susan McCue, co-chairman of the super PAC, who served as Reid’s chief of staff for eight years.

Along with McCue and Lambe, the group’s strategy is guided by Democratic consultant J.B. Poersch, who worked closely with Reid as executive director of the Democratic Senatorial Campaign Committee for three elections, as well as Varoga, who continues to operate Patriot Majority. Ty Matsdorf, a former war room director of the pro-Democratic rapid-response group American Bridge, serves as campaigns director.

The fact that Reid’s brain trust is at the helm of the Senate super PAC underscores how much leeway independent groups have in the freewheeling campaign finance landscape, said Larry Noble, a former general counsel of the Federal Election Commission. Although Reid is not on the ballot this year, as majority leader he is leading the effort to keep the Democrats in power.

“It calls into question the whole concept of independence,” said Noble, counsel to the Campaign Legal Center, which advocates for reducing the influence of big money on campaigns.

McCue said super PAC officials are careful not to discuss strategy with anyone involved in candidate or party efforts.

“Hardly a day goes by that I’m not in contact with or following the direction of our attorneys,” she said. “We do adhere to really strict compliance.”

Reid spokesman Adam Jentleson said in a statement that “there are very clear guidelines and Senator Reid observes them all meticulously.”

The Senate Majority PAC team came together in early 2011, haunted by narrow Democratic Senate losses the year before in states such as Illinois and Pennsylvania, where little outside money had been spent on the left. In the 2012 elections, the Senate Majority PAC spent $42 million and won 13 of 16 races.

As they made plans for 2014, the group’s strategists knew they faced steep odds against keeping the Senate in Democratic control. Americans for Prosperity alone is expected to spend more than $125 million on ads and field efforts supporting GOP candidates.

A key decision, Senate Majority PAC officials said, was to lay down air cover early. When Sen. Mark Pryor (D-Ark.) was pounded with ads last spring by the Club for Growth and the Senate Conservatives Fund, the Democratic operation punched back at Pryor’s Republican challenger, Rep. Tom Cotton.

“Congressman Cotton forgot about us,” declared a spot that ran in June 2013, accusing him of losing touch with Arkansans.

The sustained attacks that followed — particularly those spotlighting Cotton’s votes against the farm bill and disaster-relief measures — have taken a toll, according to Republican strategists.

“Mark Pryor was the walking dead at the beginning of this race,” said a top GOP operative in Arkansas, who spoke on the condition of anonymity to offer a candid assessment of the opposition. “Everybody on both sides had decided this was one the Democrats were going to lose. They were able to come in and keep it close.”

Similar ad blitzes around the nation have taken the pressure off Democratic candidates and the party’s Senate campaign committee, which is investing in a $60 million field effort to turn out voters.

In North Carolina, the Senate Majority PAC and Patriot Majority have spent millions defending Sen. Kay Hagan, who is locked in a tight race with Republican state House Speaker Thom Tillis. In Michigan, the groups have helped erode the standing of GOP candidate Terri Lynn Land, with ads that accuse her of answering to billionaires backing her campaign, “not to us.” In Alaska, the Senate Majority PAC has provided most of the funding for Put Alaska First, a super PAC backing Sen. Mark Begich that has spent more than $5 million.

The expansive presence of the Reid-linked groups has come at the expense of smaller, state-based efforts to protect Senate Democrats. A series of individual super PACs set up to support Hagan, Pryor and Sen. Mary Landrieu (La.) withered when big checks did not materialize, said people involved in the projects.

Chao, a Washington-based strategist with extensive experience in those states, said, “No one called me and said, ‘Don’t do this.’ But our donors just disappeared.”

Senate Majority PAC officials said they did not discourage donors from giving to other groups.

“I don’t believe people get elbowed out,” Poersch said. “I think they come in believing it’s going to be a lot easier, and it isn’t.”

The Senate Majority PAC has been boosted by the help of top Democrats, including President Obama, who headlined two fundraisers for the super PAC this summer. By the end of July, the group had raised $32 million, according to Federal Election Commission filings. It raised $6 million in August, the group said, in its best fundraising month of the year.

“It’s always going to be in the Democratic DNA to want to reject money in politics,” Lambe said. “I don’t want to say that people are comfortable giving to a super PAC, but I do think people understand that you can’t disarm.”