WASHINGTON — Mortgages with low down payments can be just as safe if other underwriting conditions are met, a federal housing regulator said Tuesday. Mel Watt, the director of the Federal Housing Finance Agency, was testifying in front of the House Financial Services Committee, after Fannie Mae FNMA, +0.47% and Freddie Mac FMCC, +0.47% both started making mortgages available to those who make down payments of just 3%. That has raised the ire of Republicans, who say the move risks a repeat of the housing bubble. “All things being equal, is a 3% down loan riskier to the taxpayer than...