Updated: 11:09 p.m.

Sen. John Ensign (R-Nev.) abruptly announced on Thursday he will resign from the Senate, effective May 3.

In a statement released by his office, Ensign cited the ongoing Senate ethics investigation stemming from his extramarital affair with the wife of his former top aide as the reason he was stepping down now as opposed to the end of 2012, when he was not seeking re-election.

“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. “For my family and me, this continued personal cost is simply too great.”

In February, the Ethics Committee hired special counsel Carol Elder Bruce to expedite its investigation of allegations related to Ensign’s affair with Cynthia Hampton, a campaign staff member married to Ensign’s former senior aide, Doug Hampton. On March 7, Ensign announced he would not run for re-election, saying “there are consequences to sin.”

Earlier this month, Doug Hampton pleaded not guilty to charges that he violated Congressional revolving-door laws by lobbying the Nevada Republican shortly after leaving his job on the Hill.

The Justice Department and Federal Election Commission had announced separately that they were not pursuing investigations into Ensign’s conduct, but the Senate Ethics panel was pressing ahead.

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

Sources said the Senator informed his Nevada and Washington, D.C., staff of his decision late Thursday afternoon.

Ensign’s imminent resignation is a major boost to Republicans, who expected a competitive 2012 open-seat race between Reps. Dean Heller (R) and Shelley Berkley (D). Heller announced his bid in March, and Berkley said she would run last week.

Nevada law dictates that Gov. Brian Sandoval (R) must appoint a successor who will serve out the rest of Ensign’s term through 2012. Nevada Republicans expect Sandoval to pick Heller because it gives the Congressman a leg up next year and the ability to essentially run as an incumbent.

Heller’s reaction to the speculation was brief. “Governor Sandoval has the authority to appoint individuals to the U.S. Senate, and I will respect whatever decision that he makes,” he said.

“I think it’s a good thing,” said Ryan Erwin, a Nevada-based GOP consultant. “Anytime you can have an incumbent, and somebody there to build a record on the Senate side, continuing the conservative voting record that he’s had in the House, it’s a good thing. He’s still going to have to work hard, he’s still going to have a tough race in 2012.”

Democrats wasted no time in attacking the presumptive Senate appointee, and the Democratic Senatorial Campaign Committee released a statement attacking Heller minutes before Ensign’s news release.

“There will be a very clear choice for Nevadans between an uncompromising extremist like Dean Heller, who wants to end Medicare and cut loans for small businesses to give more tax breaks for the very rich, and Shelley Berkley, a true fighter for Nevada’s economy and middle class,” DSCC Executive Director Guy Cecil said. “Nevada will remain a top target for Senate Democrats.”

Privately, however, Democrats acknowledged that if Heller is indeed appointed to the seat, Berkley would be at a huge disadvantage in the Senate race next year.

Senate Democrats have been publicly supportive of Berkley’s nascent Senate candidacy: DSCC Chairwoman Patty Murray (Wash.) announced last week that the committee was backing the Las Vegas-area Congresswoman — a signal that Democrats were attempting to clear the primary for her.

If Heller were chosen for the Senate seat, Nevada law states the governor has a week to call a special election within 180 days of the House vacancy. Although district lines are due to be redrawn this cycle in redistricting, the current 2nd district is highly competitive territory. The 2008 presidential race was basically a draw in the district, with Sen. John McCain (R-Ariz.) eking out a victory by fewer than 100 votes.

Steven T. Dennis contributed to this report.