((Cathleen Allison / AP))

Ensign announced last month that he would resign effective May 3 amid an ethics committee investigation into his handling of an affair with a former staffer. His farewell speech, made late in the afternoon , was heard by a lone senator presiding over the chamber.

Ensign defended his record in the Senate, apologized to his colleagues and lamented what he termed his “loss of naiveté” over the course of his political career.

“When I first ran for office back in 1994, I was rather naive,” Ensign said. “I was also very idealistic. I simply wanted to make a difference in this great country. Throughout the years, I may have lost my naiveté, but I never lost my idealism.”

Ensign warned his absent colleagues about “how dangerous the feeling of power and adulation can be” and counseled them to “surround yourself with people who will be honest with you about how you really are and what you are becoming.”

“When one takes a position of leadership, there is a very real danger of getting caught up in the hype surrounding that status,” he said. “Oftentimes, the more power and prestige a person achieves, the more arrogant a person can become. As easy as it was for me to view this in other people, unfortunately, I was blind to how arrogant and self-centered I had become.”

Two years ago, Ensign admitted that he had had an affair with Cynthia Hampton, his former treasurer. Hampton was the wife of Doug Hampton, Ensign’s former administrative assistant. Both Ensign and his wife had been very close friends with the Hamptons. In the wake of the scandal, it was revealed that Ensign’s wealthy parents had given the Hampton family $96,000.

The Justice Department dropped its investigation into Ensign last December, one month after the Federal Election Commission dismissed a separate complaint on the matter.

Ensign emphasized in his resignation statement last month that he believed he had broken no laws or Senate rules.

On Monday, he apologized to his colleagues. “I know that many of you were put in difficult situations because of me, and for that I sincerely apologize,” he said.

He also expressed his regret to two former colleagues in particular — Larry Craig (R-Idaho) and the late Ted Stevens (R-Alaska) — for having called on them to resign from the Senate amid their own personal scandals.

Ensign also mentioned his wife, Darlene, who Ensign said “has fought through so many struggles” and “is owed more than I could ever repay.”

“I do not deserve a woman like her, but I love her and I’m so grateful that the Lord has put her in my life,” he said.

Nevada Gov. Brian Sandoval (R) has appointed Rep. Dean Heller (R) to succeed Ensign in the Senate until a special election for the seat is held in November 2012.

Staff writer Paul Kane contributed to this report.