LAS VEGAS -- The Senate Ethics Committee said scandal-scarred Nevada Republican Sen. John Ensign made the right decision to turn in a letter of resignation Friday as he faced an unrelenting, but as yet unfinished, two-year probe of his conduct.

The panel's chairman, Sen. Barbara Boxer, D-Calif., and the vice chairman, Johnny Isakson, R-Georgia, issued a terse statement saying the committee had spent 22 months investigating "and will complete its work in a timely fashion."

"Senator Ensign has made the appropriate decision," the statement said.

The committee cannot take disciplinary action against Ensign once he is no longer a senator, and, with the Senate in recess, it is unlikely that the committee will be able to do so before Ensign's May 3 resignation.

The committee, however, could still issue an embarrassing statement on the propriety of Ensign's behavior after his departure and even goes so far as to recommend a criminal investigation. Committee members, however, do not have authority over federal investigators, and their request could be ignored.

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Ensign, 53, cited "wear and tear" on himself and his family in his resignation announcement Thursday.

His decision comes nearly two years after Ensign acknowledged having had an extramarital affair with a former staffer. He was accused of helping the woman's husband -- a top former Ensign staffer -- obtain lobbying work.

Ensign's pending departure also casts a new sense of urgency over Nevada's closely watched Senate race to replace him. After he announced last month that he would not seek re-election, Democrats hoped to claim the seat to protect their fragile Senate majority.

In the meantime, Republican Gov. Brian Sandoval will appoint a successor to serve the remainder of the term through the end of 2012. Sandoval previously had endorsed Republican Rep. Dean Heller of northern Nevada in the race and is widely expected to crown him an incumbent, affording Heller a slight advantage over Rep. Shelley Berkley, the Democrat's favored candidate.

Heller's appointment to the Senate, meanwhile, would require an unprecedented special congressional election in Nevada.

Because of a quirk of Nevada politics, state leaders are uncertain about how to carry out the never-enforced special election law, which does not allow for a primary. Their decision could decide the political fate of tea party favorite and perennial candidate Sharron Angle, who has been running for Heller's seat and could be closed out of the race if party leaders are allowed to pick their general election contestants.

Ensign insisted Thursday he has done nothing wrong. But he said he was shaken by the Senate Ethics Committee decision in February to name a special counsel to look into the matter, after the Justice Department and the Federal Election Commission investigated and then dropped their cases.

"I was hopeful that, with the closure of these investigations against me, the wear and tear on my family and me would soon be over. This was not the case," he said.

The timing of Ensign's announcement remains a mystery given his long insistence that he would not give up his seat.

Ensign initially said he would campaign for a third term, then announced in March he would not pursue re-election in 2012 to protect his family from campaign attacks involving his role in Doug Hampton's lobbying career. He said at the time that the Senate investigation hadn't influenced his decision.

"If I was concerned about that, I would have resigned, because that would make the most sense, because then it goes away," he said.

It was not immediately clear what, if anything, had changed since he made those remarks last month. An ethics committee official said Friday that neither a vote nor a public hearing had been scheduled in the Ensign investigation prior to his announcement.

Several national and state Republican leaders said Thursday they hoped Sandoval would appoint Heller to Ensign's seat.

"It certainly helps clear the air and narrows the field," said former Gov. Bob List, now a national committeeman. "I certainly would urge the governor to appoint Dean Heller to the seat."

Berkley and Heller had been evenly matched, with their comparable political credentials and name recognition in Nevada. Wealthy businessman Byron Georgiou is also seeking the Democratic nomination.

But choosing Heller or another Republican would give the GOP a clear, albeit slight, advantage of incumbency in a highly competitive seat that could decide which party controls the Senate after next year's elections.

In could also shape the line-up of Nevada's four House contests in a year where President Barack Obama's re-election bid could turn out votes for other Democratic candidates.

State officials don't know how they would proceed in the event of a special election.

"The vacancies that we are talking about don't even exist yet," said Robert Walsh, deputy secretary of state for southern Nevada. "So we are going to use this time to do considerable research in order to have a well-considered legal opinion as to how to proceed. We will have that at the time that the vacancy actually exists."

Ensign announced in June 2009 that he had an extramarital affair with Cynthia Hampton, a former member of his campaign staff, and that he had helped her husband, Doug Hampton, a member of his Senate staff, obtain lobbying work with two Nevada companies.

Doug Hampton has been indicted for illegally lobbying the senator's staff. Federal law prohibits a former senior Senate aide from lobbying the Senate for one year after terminating employment.

The government watchdog group that requested the Senate ethics investigation said Ensign should have resigned sooner.

"Sadly, it's not because he's seen the error of his ways, or even to `spend more time with his family," Citizens for Responsibility and Ethics executive director Melanie Sloan said in a statement Thursday. "The truth no one is likely to admit is that Sen. Ensign is being pushed out to give the Republican party a leg up in the 2012 election."