Republican mega-donors – dismayed by the mounting primary victories of billionaire Donald Trump – have turned to new strategies to thwart him, as the Super Pacs that have launched multi-million-dollar ad campaigns for other candidates have largely failed to make a dent in his poll leads or electoral success.

Although Republican presidential Super Pacs have collectively raised almost $300m since the start of 2015, they flopped in their missions to sustain the candidacies of ex-governor Jeb Bush, governors Chris Christie and Scott Walker, and others.

Super Pacs are independent fundraising and campaigning committees that, unlike official campaign committees, can raise unlimited checks – but they are not allowed to coordinate with candidates’ campaigns and must disclose their donors.

Despite fear among groups opposing the role of big money in politics, the fundraising organizations have so far signally failed to have the decisive impact in the 2016 election that many expected.

On the Democratic side, leftwing candidate Bernie Sanders – who has sharply criticized Super Pacs, although a number have not heeded his words and spent close to $2m to support him – leapt ahead of Hillary Clinton in February campaign fundraising thanks to small donors and, as he puts it in his campaign literature, “not the billionaires”.

“It’s obvious across the board that money is not the influencer it was historically,” says Fred Zeidman, a Houston-based fundraiser and donor to a Jeb Bush-supporting Super Pac who is taking a break before deciding who to back next.

Those big expensive ads didn’t have the impact we hoped they would Mel Sembler, GOP fundraiser

The failure of Super Pacs to make a crucial impact has caused lots of heartburn for GOP fundraisers and numerous mega donors. And those frustrations have only deepened with the growing strength of the billionaire Donald Trump who, flaunting his outsider shtick, has regularly bashed his rivals for their reliance on Super Pacs and big donors, while garnering enormous free media coverage with verbal bombshells.

“Those big expensive ads didn’t have the impact we hoped they would,” said Mel Sembler, a Florida shopping mall magnate who was a top fundraiser for the Jeb Bush-backing Right to Rise Super Pac which hauled in about $119m, but spent only a few million targeting Trump. “The antics that Trump puts on get him all the air time.”

Some GOP insiders fault the Super Pacs’ game plans. Until recently, little was spent by Super Pacs allied with senators Marco Rubio and Ted Cruz or others attacking Trump’s incendiary rhetoric, contradictory stances and checkered business record.

“It was a tactical error on the part of several candidates to attack each other rather than Donald,” said veteran GOP operative Charlie Black, who notes that Super Pacs backing Jeb Bush, Marco Rubio and Ted Cruz aimed much of their fire at one another.

Super Pacs were spawned by a Supreme Court ruling in 2010 that overturned decades of campaign finance laws and opened the door for corporations, unions and individuals to write unlimited checks to outside groups promoting the election or defeat of federal candidates. By law, Super Pacs are supposed to operate independently of candidate campaign committees, which can only accept limited individual contributions of $2,700.

Despite their lackluster results this year, GOP Super Pacs have far out-raised their Democratic counterparts, according to the nonpartisan Center for Responsive Politics. From the start of 2015 through this January, GOP Super Pacs supporting their presidential contenders raised $289m, data from the center reveals, while Democratic ones backing their candidates pulled in only $59m.

The disparity is striking, and partly attributable to the strong anti-Super Pac message and small donor funded campaign that Senator Bernie Sanders has been running. Sanders has repeatedly touted the importance of his small donor base which late last month reached over 4 million contributions – an average of about $30 per donation. Through February, his campaign had raised $137m, with a whopping $42m in February alone, or $12m more than Hillary Clinton’s campaign did that month.

I think the free airtime Trump has enjoyed has given him a great advantage Fred Malek, GOP fundraiser

But Clinton has a formidable and well funded Super Pac supporting her, Priorities USA Action, which has raked in $50m and, if she wins the battle against Sanders, is reportedly expected to raise $200m for the general election.

At this stage, however, the allure of big money Super Pacs has been much stronger on the GOP side, although their ineffectiveness in slowing Trump’s inexorable rise has spawned grousing and finger pointing.

Veteran fundraisers criticize the media coverage generated by Trump’s television personality and bombastic one liners.

“I think the free airtime Trump has enjoyed has given him a great advantage,” said Fred Malek, a leading fundraiser for the Republican Governors Association. “Because Trump is entertaining and says outrageous things they tend to cover him.”

Likewise, GOP operative Black quips that Trump’s free media coverage is due to the “CNN Super Pac”. A 30-minute ad by Barack Obama shown in prime time in 2008 – as some Trump speeches are today, for free – was estimated to cost his campaign well over $1m.

But with Trump’s lead growing – including wins in seven out of 11 states on Super Tuesday – fears in establishment donor circles have escalated, and some anti-Trump Super Pacs have seen seven figure donations surge. More is expected before the key primaries in Florida and Ohio on 15 March.

Days before Super Tuesday, for instance, the pro-Rubio Super Pac Conservative Solutions unleashed a multi million dollar ad drive aimed at Trump’s positions. Rubio’s allies have vowed to replicate this in coming weeks with another “multi-million” dollar blitz, according to Jeff Sadosky, a spokesman for the Super Pac. A few of its biggest donors, including hedge fund mogul Paul Singer who gave $2.5m last year, have ponied up millions more, fundraisers with ties to the Pac have told the Guardian.

The Right to Rise Super Pac spent millions backing Jeb Bush, who dropped out of the race on 20 February.

Meanwhile, the Our Principles Pac, which has received at least $3m from the family of billionaire Joe Ricketts, the founder of investment giant TD Ameritrade, spent about $2m against Trump in Iowa before their caucuses last month where Trump placed second.

“Significant resources will be spent between now and March 15,” said Brian Baker, a strategist for the Pac and a Ricketts family adviser.

The Pac, which has been running ads that hit Trump on a few issues, including fraud allegations about his now shuttered Trump University, is expected to focus heavily on Florida and other delegate rich states in a multi-million dollar ad effort. Sources say that it has roped in a few other seven figure donors including Paul Singer.

And the Club for Growth, which spent about $1m on ads in Iowa, poured another couple million into ads in two states before Super Tuesday, sources say, and is now focused on the big winner-take-all states that will vote on 15 March.

Still, some analysts note that big donor blues linger.

“There are a lot of disappointed funders out there,” says Sheila Krumholz, the executive director of the nonpartisan Center for Responsive Politics. “A lot of it has to do the fact that they dithered in taking on Trump,” while failing to boost several candidates including Bush.

Exhibit A was the Jeb Bush-supporting Right to Rise Super Pac, which was unveiled in early 2015 with enormous hype as a “shock and awe” spending vehicle: it raised over $100m dollars with a big helping hand from Bush before he officially announced his candidacy in mid 2015 – an envelope-pushing tactic since formal candidates are not legally allowed to raise unlimited checks, but only $2,700 per person. Its fundraising muscle was credited by some with intimidating 2012 candidate Mitt Romney from running again this year.

Despite their losses, some Super Pac donors say that their richer comrades aren’t fretting much. “To some people who are Super rich, $6m doesn’t mean that much,” says Stan Hubbard, a broadcasting billionaire. Hubbard, who gave $10,000 to a Super Pac backing Christie, was referring to a $6m donation made to the Pac by hedge fund multi-billionaire Steve Cohen.

Hubbard is one of many big donors who have already ponied up to more than one Super Pac, in part because of the record crop of 17 candidates that the GOP started out with last year: Scott Walker was Hubbard’s first choice and he wrote a $50,000 check to his Pac. And recently Hubbard kicked in $10,000 more to the anti-Trump Our Principles Pac.

To be sure, Super Pacs early on seemed to have a magnetic attraction to GOP candidates and many big donors.

Despite some donor fatigue other donors will double down and more donors are likely to step forward Sheila Krumholz, Center for Responsive Politics

“A Super Pac can be underwritten by a handful of big donors and keep candidates in the race even though they don’t have that much political support,” said Black. “They’ve given opportunities to long shot candidates to get in and run for the full year leading up to Iowa and New Hampshire.”

Some big fundraisers voice related concerns. “I think having a handful of people making outsized contributions is not what the founders had in mind,” said Malek. “But the law is the law.”

Notwithstanding their lackluster results so far, campaign finance analysts say that some Super Pacs this cycle have pushed the fundraising envelope, in part due to the Federal Election Commission’s lax enforcement of rules requiring Super Pac disclosure of all donations, and barring coordination between the Pacs and campaigns. “We’re seeing more bold initiatives in terms of end runs around disclosure and coordination [rules], and a general sense they can operate with impunity,” says Krumholz of the nonpartisan Center.

Looking ahead to the general election, Krumholz adds, Super Pacs for presidential candidates will likely be major players and set spending records if current trends continue. Krumholz predicts that this year’s spending by presidential Super Pacs “will likely exceed those of 2012, when single candidate Super Pacs spent more than $239m”.

She adds: “Despite some donor fatigue other donors will double down and more donors are likely to step forward.”