A pair of largely overlooked U.S. House races in the Maryland suburbs of Washington, D.C. are racing up the leaderboard of the most expensive congressional primaries in U.S. history.Why it matters: The massive volume of cash pervading these elections has some candidates raising the possibility that there is such a thing as too much spending for your favored candidate.Voters "see the frequency of the ads, and now they're asking questions," said Wala Blegay, a Prince George's County Council member running in Maryland's 5th district.Said former Capitol Police officer Harry Dunn, another candidate in that race: "It's tough to square, like, why is someone donating this much money?"Driving the news: Outside groups — most notably the pro-crypto super PAC Protect Progress and AIPAC's United Democracy Project — had spent a collective $12.5 million in MD-05 as of Monday, according to FEC filings.All the actual Democratic candidates in the primary spent a combined $10.5 million as of June 3.More than 20 Democrats are running in the primary to succeed former House Majority Leader Steny Hoyer (D-Md.).Data: Federal Election Commission; Chart: Andrew Solender/AxiosBy the numbers: The super PAC spending has almost all favored state Del. Adrian Boafo, who is backed by Hoyer, Maryland Gov. Wes Moore and Sen. Angela Alsobrooks.Hoyer's AmeriPAC, Congressional Black Caucus-aligned Rolling Sea Action Fund and American Bridge-affiliated Project 218 have also spent six-figure sums supporting Boafo.Just one group — Servant-Leader Fund, which supports Democratic veterans running for office — has spent for another candidate, putting down $135,000 to support former state Del. Rushern Baker.The top-spending candidates in the race, as of the beginning of June, included businesswoman Quincy Bareebe and Dunn, according to FEC filings.Bareebe had spent $5.1 million — though she has given her campaign $5.7 million in personal loans.Dunn, a darling of the liberal grassroots endorsed by former House Speaker Nancy Pelosi (D-Calif.), had spent $3.5 million during that period, most of which came from grassroots donations.Boafo's own campaign had spent just $830,000 by June 3, the filings show. Blegay spent a little under $90,000.What they're saying: Dunn said of the outside spending in an interview with Axios, "I've been one of the top fundraisers in the country, and I can't compete with that. It's tough to overcome.""The entire experience has been so frustrating because almost none of the ads that people have seen on TV have said 'Adrian Boafo for Congress,'" said Blegay. "They're all ads from Protect Progress, United Democracy [Project].""People are seeing them about every hour," she added. "It doesn't take $12 million to win this race, and that's just how much money they've put in."Boafo's campaign did not respond to repeated requests for an interview.On the other side of D.C.'s suburbs in Maryland's 6th district, Rep. April McClain Delaney (D-Md.) is trying to withstand a brutal primary challenge from her predecessor, David Trone.Trone, the wealthy founder of Total Wine & More who vacated the seat last cycle to launch a failed bid for Senate, has given his campaign a staggering $25 million in loans.McClain Delaney, whose husband John Delaney is also a wealthy businessman who previously represented the seat in Congress, has loaned her campaign $7.4 million.Protect Progress has come in during the final stages of the race with more than $500,000 in spending favoring McClain Delaney.The intrigue: McClain Delaney and Trone are both considered moderate, establishment-aligned Democrats.Several other Democrats are running in the primary, such as progressive former Consumer Financial Protection Bureau official Alexis Goldstein, but they have raised paltry sums by comparison.McClain Delaney and Trone have been running ads playing up their progressive bona fides and whacking each other as insufficiently anti-Trump, anti-ICE or pro-abortion.