Rep. Barney Frank (D-Mass.), who is awaiting a report from the House ethics committee on his involvement with a male prostitute, has received some welcome news: His most serious Republican challenger has withdrawn.

James Nuzzo, a physician who served as a policy adviser to then-Vice President Bush, announced Wednesday he was pulling out of the 4th District race "for personal reasons." His GOP primary campaign was launched five weeks ago with the backing of state party leaders and former U.S. attorney general Elliot L. Richardson, his honorary chairman.

Nuzzo's withdrawal leaves the GOP field to conservative John Soto, an accountant who has never won elective office. Soto, who reportedly has raised only $15,000 for the race, drew attention by taking a test for the AIDS virus, passing and challenging Frank, who is gay, to take one.

Frank, an outspoken fifth-term liberal who represents suburbs west and south of Boston, was considered vulnerable because of the House probe into his relationship with Stephen Gobie. But no Democrats challenged him, and Soto has had difficulty establishing himself as a serious alternative. A state GOP official said yesterday that the party had not decided whether to aid Soto.

Frank called Nuzzo's decision "positive . . . because I assume the major motivation is he thinks he couldn't beat me."