Participation took off. For the week that ended June 15, the Mortgage Bankers Association said HARP accounted for one-fifth of all refinancing applications. Earlier this spring, it was as high as one-third.

Borrowers can't shop around for just any lender, though. The biggest mortgage servicers, including Bank of America, Wells Fargo and JPMorgan Chase, accept HARP applications only for loans in their own portfolios.

That largely cuts out smaller mortgage bankers, who sell all or most of the loans they make.

Douglas Schukar, president of USA Mortgage in Creve Coeur, said his firm has the largest share of the St. Louis mortgage market, but only rarely is able to handle a HARP loan.

“It's a government-mandated program that the consumer is supposed to benefit from, and we don't have access to it for our customers,” Schukar said. “You have to turn them away if you can't offer it to them.”

Schukar says the big mortgage servicers are taking a similar approach to the Federal Housing Administration's Streamline program, which also aims to help people who owe more than their house is worth.