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Then came the shocking fall. During a 2009 news conference, Mr. Ensign revealed he had engaged in a nine-month affair with a staffer. Cindy Hampton’s husband also worked for Mr. Ensign and was a good friend.

“If there was ever anything that I could take back in my life, this would be it,” Mr. Ensign said at the time.

Amid the scandal, Mr. Ensign’s parents provided the Hamptons with $96,000 that they described as a gift and Mr. Ensign helped find Doug Hampton a lobbying job.

The Justice Department and the Federal Election Commission investigated the alleged bribes and then dropped the cases with little explanation. The Senate ethics committee, however, recently named a special counsel to look into the allegations.

Mr. Ensign has spent nearly $700,000 in legal fees, leaving him with barely $300,000 in campaign cash. He hopes to raise $1 million by July.

But for all his bravado, Mr. Ensign, too, predicts his journey toward redemption will be rocky.

“Campaigns are always ugly. This one is going to be exceptionally ugly,” he said when questioned in Washington recently. “I’m not under any illusions that this is going to be easy. I know it’s going to be unbelievably hard.”