Embattled Sen. John Ensign (R-Nev.) announced Thursday night that he will resign from office in early May, a move that comes amid an ethics investigation into his conduct.

“While I stand behind my firm belief that I have not violated any law, rule, or standard of conduct of the Senate, and I have fought to prove this publicly, I will not continue to subject my family, my constituents, or the Senate to any further rounds of investigation, depositions, drawn out proceedings, or especially public hearings,” Ensign said in a statement posted on his Web site. “For my family and me, this continued personal cost is simply too great.”

Ensign’s resignation, which was first reported by Las Vegas Sun columnist Jon Ralston, will be official May 3 and follows by six weeks his announcement that he would retire at the end of his term, which expires in 2012.

The Senate Ethics Committee is investigating Ensign’s handling of an affair with a former political aide whose husband was also a top legislative aide to the senator.

Earlier this year, the committee hired outside counsel to begin a more formal phase, which probably would have led to a public hearing on formal allegations against the senator or the public release of its allegations.

With Ensign gone from the Senate, the Ethics Committee will have no jurisdiction in the matter and probably will keep private the results of its 20-month investigation.

In a statement Thursday night, Senate Ethics Committee leaders indicated that they would complete their investigation despite Ensign’s departure. They also suggested that their findings might have prompted him to resign.

“The Senate Ethics Committee has worked diligently for 22 months on this matter and will complete its work in a timely fashion,” wrote Chairman Barbara Boxer (D-Calif.) and Vice Chairman Johnny Isakson (R-Ga.) said. “Senator Ensign has made the appropriate decision.”

In June 2009, Ensign publicly admitted that he had an affair with Cynthia Hampton, who was his political treasurer and was married to Doug Hampton, Ensign’s administrative assistant.

In 2008, Ensign dismissed both Hamptons from his payroll. Ensign’s parents, wealthy casino magnates, paid the Hampton family $96,000 in what was labeled gift income.

Doug Hampton returned to Las Vegas and began working as a lobbyist for a consulting firm run by Ensign’s top political advisers. He has alleged that Ensign helped him line up his first few lobbying clients — all donors to Ensign’s political committees — and that the senator helped arrange meetings for him with Obama administration officials.

A Justice Department investigation into possible criminal activity by Ensign ended with no charges. Doug Hampton was indicted this year on charges of lobbying the Senate less than a year after leaving the chamber.

Ensign’s resignation would allow Gov. Brian Sandoval (R) to appoint a replacement to serve until the seat is filled in the 2012 election. Rep. Dean Heller, a Republican who is campaigning to succeed Ensign, is the front-runner for the appointment .

If Heller is named to the Senate, there will be a special election to replace him in the sprawling 2nd District. Nevada election law is vague, and officials in the secretary of state’s office are currently researching the rules. In a free-for-all race, Sharron Angle, the 2010 Senate challenger to Harry M. Reid (D), would be considered the early favorite. If state party committees pick the candidates, Angle has less chance of being the Republican nominee.

Ensign’s resignation will close the book on a once-promising political career that began when he was elected to the House in 1994. Ensign had begun to lay the groundwork for a possible presidential bid in 2012 when the news of his affair with Cynthia Hampton broke.

Staff writer Aaron Blake contributed to this report.