Nevada Sen. John Ensign, facing an ethics investigation stemming from an affair with a campaign aide, will resign his seat on Friday, his office announced Thursday afternoon.

The Republican had announced in March that he would not seek a third term in 2012, saying he wanted to spare his family from an "exceptionally ugly" campaign.

The decision now to resign with more than 20 months left in his term would spare him prosecution by the Senate Ethics Committee, which in February named a special counsel to investigate potential violations of ethics rules and federal law.

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

Ensign will formally submit a letter of resignation to Vice President Biden — also president of the Senate — on Friday.

Two Nevada members of Congress — Democrat Shelley Berkley and Republican Dean Heller — have already entered the race to succeed Ensign. Nevada's new Republican governor, Brian Sandoval, could potentially appoint Heller to replace Ensign, giving him the potential advantage of incumbency in what both parties expect to be one of the cycle's premiere races.

A spokesman for Sandoval said the governor has not yet been notified of the senator's decision.

Democrats hold a slim 53-47 majority in the Senate, and consider the Nevada seat one of their top pickup opportunities.