WASHINGTON — In the past five years, more than 3 million borrowers who had little or no equity in their homes refinanced into cheaper mortgages using a federal program that’s been widely hailed as a success.

Yet there remain 676,000 loans eligible for the Home Affordable Refinance Program (or HARP), according to federal estimates, and neither the regulators nor the lenders can figure out how to get these homeowners to bite before the program expires at the end of 2015.

The Federal Housing Finance Agency, which oversees mortgage finance giants Fannie Mae and Freddie Mac, is trying. It’s reinvigorating its push to reach the homeowners who can benefit, starting with a kick-off event Tuesday in Chicago, where regulators and housing advocates gathered in a town hall-type setting to pitch HARP to area residents.

Mel Watt, who took over as head of FHFA in January, said that homeowners who refinance through HARP save an average of $191 a month. But people are not stepping forward to cash in on the savings, Watt told the Chicago audience.

“We have written to them. We have called them, and they’re saying this is too good to be true,” said Watt. The housing meltdown has made homeowners wary of scams, he said. They don’t believe the deals offered through HARP, which caters only to borrowers who are current on their mortgages, yet the government is itching to reach these hesitant borrowers, Watt added.

“You are the most reliable borrowers we have because even in the worst of the crisis you have continued to pay your mortgage on a regular basis and we want to reward you if you would just come and say: ‘Yes. I would like to take advantage of HARP,’ ” Watt said.

But experts who track the issue say getting more people to refinance through the program may be a hard sell. They say the low-hanging fruit has been picked and the rest who are eligible are either suspicious of the program or simply not interested.

Lenders have sunk lots of money into marketing HARP to eligible borrowers, said David Stevens, chief executive of the Mortgage Bankers Association. “They’re pulling out all the stops to get the phones to ring, but the response is still low.”