WASHINGTON (Reuters) - Freshman Democratic Representative Eric Massa announced his resignation on Friday, saying an ethics investigation into alleged misconduct toward a male staff member would tear his family and staff apart.

"In the incredibly toxic atmosphere that is Washington D.C., with the destruction of our elected leaders having become a blood sport, especially in talk radio and on the Internet, there is also no doubt that an ethics investigation would tear my family and my staff apart," the New York lawmaker said in a note posted on the Internet, massa.house.gov/.

His resignation is effective on Monday at 5 p.m., he said.

Massa was the second Democrat in a week to be hit by damaging ethics questions, and his departure is a further embarrassment for House Speaker Nancy Pelosi, who had pledged to create “the most honest, most open and most ethical Congress in history.”

Republicans have ridiculed Pelosi over this week’s ethics revelations, saying the behavior of Democrats had made a mockery of her promise to clean up Congress.

Massa announced earlier this week that he would not seek re-election because of health reasons. News reports said he was under an ethics investigation for alleged sexual harassment against a male staffer.

“My difficulties are of my own making,” he wrote. “I am also aware that blogs and radio will have a field day with this in today’s destructive and unforgiving political environment.”

Massa’s problems follow an ethics probe of fellow New York Representative Charles Rangel, who was admonished by the House ethics panel for taking corporate-paid trips to the Caribbean in violation of the chamber’s gift rules.

Rangel gave up his chairmanship of the powerful tax-writing Ways and Means Committee this week pending other ethics investigations.

Massa’s departure brings the number of Democrats in the U.S. House of Representatives to 253, with 178 Republicans.

His departure is not likely to have a big impact on Pelosi’s ability to round up votes to get a final healthcare overhaul bill to President Barack Obama.

Massa voted against the bill that passed the House in November in a close vote. Four vacancies bring the number of House members to 431, which means 216 votes will be needed to pass the sweeping healthcare overhaul.