Two Democratic lawmakers are calling on Fannie Mae and Freddie Mac to relax recently tightened standards for mortgages on new condominiums, saying they could threaten the viability of some developments and slow the housing-market recovery.

In March, Fannie Mae said it would no longer guarantee mortgages on condos in buildings where fewer than 70% of the units have been sold, up from 51%. Fannie Mae also won't purchase mortgages in buildings where 15% of owners are delinquent on condo association dues or where one owner has...