With the power balance in Congress at stake in this year’s midterm elections, the GOP money machine kicked into high gear. Spending on advertisements and drumming up votes was fueled by hundreds of millions of dollars from the party’s mega-donors and Super Pacs. Many donors’ spending figures marked new records.

Their return on investment, however, is probably not what they had hoped: some donors who spent eight figures notched zero wins in the Senate, while others spent far more money on losing candidates than winners. In the midterms, some of the biggest losers were Republican donors.

Among the clearest of those losers is Mehmet Oz, who self-funded much of his own failed run for office – loaning his Pennsylvania US Senate campaign about $22m, or about 55% of the roughly $40m he raised.

Meanwhile, candidates backed by Peter Thiel, the rightwing tech investor hyped pre-election as a new GOP “kingmaker”, lost in Arizona and Washington, calling into question his judgment and contributions’ value.

Other mega-donors and Pacs came out behind despite spending hundreds of millions of dollars collectively on multiple candidates who lost, according to Open Secrets, a campaign finance watchdog, and federal campaign records. Among those is Mitch McConnell’s Senate Leadership Fund Super Pac, which spent $239m; the billionaire financier Jeff Yass, who spent $47m; the hedge fund manager Ken Griffin, who spent $67m; the packaging giants Elizabeth and Richard Uihlein, who spent $77m; and Blackstone CEO Stephen Schwarzman, who spent $34m.

By contrast, Democratic candidates in high-profile Senate races generally had a larger share of small donations. GOP mega-donor and Super Pac money couldn’t overcome weak candidates that many swing voters viewed as extreme.

The results highlight that “candidate strength matters”, said Gunner Ramer, political director at Longwell Associates, a conservative communication firm. “Voters have real concerns over crime, inflation, gas prices and the economy … but all these really poor candidates – these crazy, extreme Republicans – got beat up hard.”

In Pennsylvania, Oz’s self-funding functioned as a double-edged sword that benefited him in the primaries and made him attractive to GOP base voters “who still think that you can buy a race”, said Sam Chen, a political strategist in Allentown. But once in the general election, Chen said, it meant that Oz received relatively few small donors in part because he was viewed as a self-funder.

Still, he wasn’t alone: McConnell’s Pac put up $47m. That combined with Oz’s personal spending accounted for nearly half of the stunning $140m Oz forces spent in the campaign. Two largely billionaire-funded single-candidate Pacs also went all in on Oz: Honor Pennsylvania spent about $15m, and its largest donor was Citadel CEO Ken Griffin, who gave it at least $8.8m; and American Leadership Action Pac, funded by Wall Street tycoons or mega-donors like Susquehanna International Group CEO Jeff Yass, Blackstone CEO Stephen Schwarzman and Actua CEO Walter Buckley, dropped another $15m.

Though the outside spending in Pennsylvania set a new record, Oz was a “uniquely weak candidate”, Chen noted, and his failure highlights how wealth and Super PAC money “is not the end all be all”.

“Small dollar donations, the grandma who writes you a $5 check, they are locked in and voting for you … and they are probably the type of person who tells their neighborhood, their soccer mom group, their bible study that they gave you contributions,” Chen said. “Those contributions mean a lot more.”

In Arizona, Thiel spent at least $17.5m backing Blake Masters’ failed US Senate bid, while Thiel’s Pac, Saving Arizona, which received significant funding from mega-donor Richard Uihlein, spent at least $21.5m.

Thiel’s potential to become a powerbroker was the subject of intense media attention in part because he funded a breed of rightwing populist GOP candidate that broke with the party establishment. Voters, however, were “completely repelled” by Masters, Ramer said, and though Thiel had success in primary races across the country, his money couldn’t overcome swing voter skepticism in the Arizona general election.

McConnell’s Senate Leadership Fund ended with a mixed record, but spent far more on losing races. Data released just ahead of the election by a marketing industry analyst found McConnell had shelled out $178m for advertising in five states – New Hampshire, Pennsylvania, Nevada, Georgia and Ohio.

Campaign finance records show the Senate Leadership Fund spent nearly $140m in four of those five states in which GOP candidates did not win, though Georgia is yet to be decided.

The Club For Growth Super Pac, one of the nation’s most prolific outside spenders, also fared poorly. Its primary funders were Uihlein and Yass, who put at least $46m into the Pac. It backed Masters with over $7m and spent $15m in Nevada attempting to unseat Democratic senator Catherine Cortez Masto. It also spent $12m total on winning campaigns in Ohio and North Carolina.

Meanwhile, the Sentinel Action Fund spent over $10m in Nevada and New Hampshire, and didn’t put any money in winning Senate races. Its primary funder is Tim Mellon, grandson of banking tycoon Andrew Mellon, who spent about $40m during the election cycle.

Focus groups run by Longwell Associates found Pacs’ ads were probably ineffective because voters didn’t like the Trump-backed, extreme GOP candidates that the Pacs supported – such as Adam Laxalt in Nevada, Don Buldoc in New Hampshire or Masters.

“You can hit Catherine Cortez Masto on gas prices and tie it to Joe Biden, but at least a meaningful slice of voters just were not buying it,” Ramer said. “At the end of the day, if they don’t like the Republican candidate, and it becomes a lesser-of-two-evils thing, then it may not move the votes the way that Club for Growth was hoping, and that is a reflection on Adam Laxalt.”

Many of the mega-donors’ spending totals come with a caveat. They may not include all the donors’ contributions, and Pac records may omit spending by some individuals altogether. Pacs are required by law to disclose their donors, but more are shielding their contributors’ identities by exploiting a loophole that allows donors to give to a Pac’s affiliated dark money nonprofit, which does not have to disclose its donors. The nonprofit then gives those donations to the Pac, circumventing disclosure laws.

Pacs only “have the facade of being transparent”, said Sheila Krumholz, executive director of Open Secrets, adding that the loophole adds another layer of uncertainty to the nation’s already murky campaign finance disclosure laws.

Regardless, the money means little to the GOP if the party continues nominating extremist and Trump-backed candidates in swing states, Ramer said. “The gap between what it takes to win in a Republican primary and what it takes to win in November is continuing to grow, and that is a difficulty the party will be dealing with in future elections.”

Whether the funders will have a change of heart is another matter. Republican mega-donors “clearly have money to burn and they may lose, and they may be dissatisfied with their return on investment, but they are also clearly risk takers – and it’s a low risk to them because of how much money they have”, said Krumholz. “It shows the limits of their money.”