A constellation of left-leaning nonprofits and super PACs are raising tens of millions of dollars to pave the way for Hillary Rodham Clinton’s presidential campaign — and nearly all of them have paid Mary Pat Bonner a cut.

Over the past several years, the groups, which include American Bridge 21st Century, Media Matters for America, and the super PAC Ready for Hillary, have paid Bonner’s consulting firm in excess of $6 million to help them cultivate wealthy donors and raise money, according to tax filings and campaign disclosures.

Bonner’s contracts give her firm a commission, typically 12.5 percent, on any money she brings in. Her tenacity, ties to wealthy givers, and mastery of donor-stroking have made Bonner, 48, among the most successful practitioners of a trade that is virtually invisible to voters but has taken on immense power and influence in the post-Citizens United world.

Almost every candidate for high office must now court ultrarich donors to finance their super PACs. And with each party more reliant than ever on networks of outside groups to supplement its advertising and opposition research, fundraisers like Bonner hold the keys to the big-money kingdom.

"The Bonner Group gets us the best fundraising product for the lowest cost," said David Brock, the founder of the monitoring group Media Matters and the super PAC American Bridge. "In my experience, the commission incentivizes the fundraiser to meet the ambitious goals we set."

But the growing influence of paid fundraisers has angered donors in both parties, who are skeptical of Washington’s consultant class and the secret, often lucrative deals they reach with campaigns.

Some organizations, like Freedom Partners, overseen by the conservative billionaires Charles and David Koch, emphasize their reliance on salaried staff members to raise money.

"I want my money to go to the candidate, to get them elected; I don’t want it to go to middlemen," said Andrew Sabin, a prominent Republican donor.

Several Republican presidential contenders are now courting Spencer J. Zwick, Mitt Romney’s finance chairman in 2012. But some former donors grumble about the fundraising fees paid by Romney’s campaign committees to limited liability companies established by Zwick: about $34 million, according to campaign disclosure reports.

In an interview, Zwick declined to describe his own fee. But he said that the bulk of the payments collected by the companies were in turn paid out to more than 50 other fundraisers employed by the campaign.

"We raised more money than has ever been raised before at a better cost of fundraising than has ever been done before," Zwick said.

But few fundraisers seem to command commissions as generous as Bonner’s. Political fundraisers are typically paid monthly retainers, which can reach $25,000 a month during campaigns. The Bonner Group is paid almost exclusively on commission, a practice that is legal but frowned upon by some fundraising consultants, who say it leads to fights with clients and other consultants over credit and is considered unethical by the Association of Fundraising Professionals, a national professional organization.

"I think it’s a breach of fiduciary responsibility to pay fundraisers on commission," said Cindy Darrison, a professor at the George H. Heyman Jr. Center for Philanthropy and Fundraising at New York University.

Allies say Bonner and her 20-member firm are worth the expense. The Bonner Group maintains a database of 70,000 donors and collects detailed information on their past giving, their families, and their political relationships. Many praise her energy and personal touch: thank-you notes, for example, or tickets to Broadway shows.

"Without Mary Pat, we would never be where we are today," said Craig T. Smith, a senior adviser to Ready for Hillary. Smith said the group had paid Bonner and some other fundraisers a single-digit percentage of money raised.

Bonner, who cut her teeth as a campaign aide and fundraiser for former Vice President Al Gore, is also known among colleagues for her aggressive tactics. During the 2012 campaign, Bonner, who was raising money for American Bridge, clashed repeatedly with other Democratic super PACs over joint fundraising efforts.

Early in the cycle, American Bridge wanted a larger portion of shared fundraising so that it could begin tracking and researching Republican candidates. The other groups felt that Bonner was seeking to establish her client as a central financial clearinghouse for other Democratic groups.

Several recalled attending a meeting at American Bridge where they glimpsed a half-erased whiteboard diagram, showing money flowing into American Bridge and then back out to their super PACs.

Opacity surrounds political fundraising. Priorities USA Action, a Democratic super PAC that is now preparing to back Clinton, employed several consultants to bolster its fundraising efforts in 2012. But a scan of the group’s disclosure reports shows mostly regular, round-number payments to them.

After The New York Times asked about payments to several specific fundraisers, a spokesman confirmed that the payments constituted commissions to three of them. One, Andrew Korge, a Florida fundraiser, was paid a 10 percent commission on a single million-dollar check. Another, Janet Keller, based in California, was paid a 5 percent commission on checks from a few wealthy donors totaling more than $2 million.

Irwin M. Jacobs, the billionaire co-founder of Qualcomm, said in an email that Keller had merely helped arrange for him to meet with two Priorities officials. "I was not aware that consultants might be paid a percentage of the political contributions that they raise," Jacobs wrote.

In an email, Bonner said she routinely disclosed to donors that she was being paid on commission. "We charge all of our clients the same way so there is no incentive for anyone in the firm to focus on one client more than another," Bonner said.

But there is little question who her biggest client is. Brock’s growing empire, now composed of about 10 interlocking PACs and nonprofits, uses the Bonner Group for all of its development efforts.

Two years ago, reflecting her expanding role in Brock’s enterprises, Bonner moved her company and staff into his headquarters, though she continues to serve other clients. She and Brock have adjoining offices and even share a summer rental in the Hamptons.

Brock credits Bonner with helping persuade donors that media monitoring and opposition research deserve large-scale financial support. His groups bought in more than $28 million in 2014, entitling Bonner’s firm to about $3.5 million in fees. Her commission represented his entire fundraising overhead, Brock said, which compared favorably with that of other nonprofit groups.

He also emailed a statement from 40 donors attesting to the value her firm provided.

Not everyone seems convinced. Bonner’s fees have been a perennial source of controversy in the Democracy Alliance, a club of wealthy progressive donors, each obligated to contribute money to a select roster of liberal research and advocacy organizations.

Bonner originally worked there as a consultant, helping recruit new members. Later, when she moved to take on some of the funded organizations as clients, the alliance asked that contributions earmarked by its donors be exempt from Bonner’s commission. Eventually, the Alliance ended her consulting arrangement. But an Alliance official said that there was no formal policy in place and that its staff had no way of tracking Bonner’s commissions.

Bonner said in an email that she abided by the request. She continues to attend the alliance’s private donor conferences, however, as an unpaid "donor adviser" to Marcy Carsey, a prominent Hollywood producer. Current and former executives at liberal nonprofits complain about a perception that hiring Bonner would improve their chances of being included in the Alliance’s investment portfolio.

One Alliance donor, the billionaire Boston investor Vin Ryan, said that he had not been informed of Bonner’s commission before donating to Media Matters and later demanded a written guarantee from the group that his contributions would be exempt.

"I don’t know what her role in the DA is at this point, nor do I know who she actually is a donor adviser to, nor do I know what organizations she represents within the group of organizations who we are supporting," Ryan said. "I think it’s outrageous."