Americans for Prosperity, the advocacy group backed by the billionaire industrialists Charles and David Koch, is emerging as the most powerful ally of the Republican Party ahead of the 2014 midterm elections, bombarding Senate Democrats with early air attacks that could help the GOP retake the Senate.

The organization is set to announce on Thursday a three-week ad campaign against Sen. Mark Pryor (Ark.), bringing to five the number of vulnerable Senate Democrats in its cross-hairs. It also is going after Democrats running for open Senate seats in Michigan and Iowa and running ads against House Democrats, including Rep. Nick Rahall (W.Va.).

The group’s expansive efforts underscore its ascendancy as a force in American politics and illustrate how the traditional domain of political parties has been overtaken by independent groups.

The National Republican Senatorial Committee has made just one small ad buy, last summer in Louisiana, while Karl Rove’s super PAC, American Crossroads, buffeted by weak fundraising, is participating in only one Florida special election right now.

As of this week, Americans for Prosperity has spent more than $27 million on ads since August, putting it on pace to far outstrip its overall $38.5 million budget for the 2010 midterms.

View Graphic Inside the Koch-backed political donor network

AFP’s decision to launch front-line attacks against Democrats while avoiding the Republican intraparty fights represents a new role for the organization, a powerful tea party ally that often has been at odds with the GOP leadership.

Its strategy serves as a sharp contrast with that of other tea-party-aligned organizations, such as FreedomWorks and the Senate Conservatives Fund, which are tangling with establishment GOP groups in Republican primaries.

AFP is steering clear of those battles, keeping its sights trained on Democrats such as Sens. Kay Hagan (N.C.) and Jeanne Shaheen (N.H.), providing early firepower for Republicans in their effort retake the Senate. Many of AFP’s ads feature women speaking in personal terms about the damage they say has been wrought by President Obama’s health-care law, an approach that could help the GOP peel away critical female supporters.

More Senate and House targets are still to come, said AFP President Tim Phillips, adding, “We’re definitely not finished.”

AFP’s air power has caught many of its targets off guard. Rep. Bruce Braley (Iowa), who is running to replace retiring Sen. Tom Harkin, is no stranger to attacks by third-party groups, but he was still taken aback when AFP spent nearly $500,000 against him last month.

“We knew they were going to get in early and bring a lot of resources to the table, but it was still surprising how early in the election they are spending all this money,” Braley said.

In Michigan, AFP is in the midst of a $1 million three-week blitz against Rep. Gary Peters, who is running to replace retiring Sen. Carl Levin.

“It’s difficult to respond,” Peters said. “Ultimately, the only way you can effectively respond is having $1 million yourself. That’s an awful lot of money to spend 10 months before election day.”

AFP’s aggressive incursions have alarmed Democratic strategists, who warn that the group’s assaults must be answered in kind.

“It’s unprecedented,” said Ty Matsdorf, a spokesman for the Democratic super PAC Senate Majority PAC, which has spent a little more than $2.75 million in response. “It means that groups like ours have to go up early as well. We can’t let those attacks go unchallenged.”

Democrats have frantically sought to raise money to combat AFP by highlighting the group’s ties to the Kochs. The tax-exempt organization serves as the primary political arm of an elaborate network of nonprofit groups funded by the conservative brothers and their network of allied donors. More than $44 million of the $140 million the organization raised in the last cycle came from Koch-linked funds, a Washington Post investigation found. Unlike political committees, the nonprofit groups are not required to disclose their donors.

Senate Majority Leader Harry M. Reid (D-Nev.) went after the Kochs last week during remarks on the Senate floor, saying, “We have two brothers who are actually trying to buy the country.”

Koch spokesman Robert Tappan said in a statement that AFP is “a separate, independent and distinct organization from Koch Industries.”

Of the Democrats who say that the Kochs are behind AFP’s ads, Tappan said, “They have no real message or ideas, so they attack Koch.”

Since it began in 2004, AFP has steadily expanded its national presence and its impact on state and federal politics, through on-the-ground organizing and hard-hitting television spots — usually “issue ads” that stop short of calling for the election or defeat of a candidate. The group helped fuel the Republican takeover of the House in 2010 with spots attacking vulnerable Democrats on energy and spending. And it was one of the first groups to begin running air attacks against Obama in 2012.

This cycle, AFP, which can raise unlimited sums, has used its extensive resources to begin running ads earlier than ever before — going on the air in key states even before the botched rollout of the health-care law’s Web site.

Phillips said the organization still does not hesitate to go after Republicans, pointing to its criticism of the GOP in recent months over the farm bill and a compromise budget measure.

He declined to say whether the group will run ads against Republicans this year but said AFP’s primary focus is on those who backed the Affordable Care Act.

“Obamacare was a situation where not a single Republican voted for it,” he said.

Phillips said AFP’s strategy is not built around the 2014 midterms but is a long-term effort to ultimately repeal the health-care law.

“To those who would say, ‘This is some kind of effort focused on an election,’ they disregard the five years we’ve done focused on Obamacare,” he said. “It’s been a long, detailed, grind-it-out effort for us.”

The issue is also a politically potent one, particularly in conservative-leaning states such as Arkansas, where Pryor has been on the defensive for his support of the law. AFP now plans to spend $610,000 running a statewide TV spot stressing Pryor’s embrace of the legislation.

Until now, he had been spared from AFP’s air fusillade. But the group’s large network of supporters has been promoting his backing of the law at grass-roots events, said Jason Cline, the group’s Arkansas state director.

Conservative activists have been eagerly waiting for AFP to engage against Pryor on the air, Cline said, adding that the Senate contest is the dominant race in the state: “They’re all pretty fired up about it.”

Alice Crites contributed to this report.