The Senate ethics committee on Thursday took the rare step of asking federal agencies to investigate a former colleague, saying it found “substantial and credible evidence” that Nevada Republican John Ensign broke federal laws while trying to cover up an extramarital affair with a political aide.

The panel presented its case against Ensign in blunt language delivered on the Senate floor, suggesting that his alleged violations could lead to formal charges from the Justice Department. It was the first time since 1995 that the committee had referred a case about a current or former senator to federal investigators.

Committee Chairman Barbara Boxer (D-Calif.) said the evidence was “substantial enough to warrant the consideration of expulsion” had Ensign not resigned in early May.

In a 75-page report, a special counsel hired by the committee painted a stark portrait of a narcissistic lawmaker whose affair with his wife’s good friend spiraled into a tale of devastated families, payments to buy silence and the ultimate downfall of a rising political figure.

The aide, Cynthia Hampton, has since filed for divorce, begun bankruptcy proceedings and started to work for a Christian organization. Her husband, Douglas Hampton, is under federal indictment. Ensign, who portrayed himself as a devout Christian conservative, resigned from the Senate rather than testify under oath, ending a political career that once carried presidential ambitions.

On Thursday, his attorneys denied that their client had broken any laws or Senate rules.

“Senator Ensign has admitted and apologized for his conduct and imposed on himself the highest sanction of resignation. But this is not the same as agreeing that he did or intended to violate any laws or rules,” Abbe Lowell and Robert Walker said in a statement.

A Justice Department aide would say only that the ethics report had been received.

Boxer and Sen. Johnny Isakson (Ga.), the ranking Republican on the evenly divided six-member committee, cited eight findings in which there was substantial evidence that Ensign committed legal violations or broken Senate rules.

The most prominent accusation was that Ensign broke federal “cooling-off” laws when he helped Douglas Hampton, who had worked as an aide to the senator, gain lobbying employment and clients. Ensign dismissed both Hamptons after the affair became public.

The Justice Department’s public integrity unit indicted Douglas Hampton in March on charges that he broke a one-year ban on lobbying his former Senate boss.

The committee also alleges that Ensign engaged in “potential obstruction of justice” in the case, saying he had deleted e-mails after the panel began its investigation.

The report also alleges that $96,000 that Ensign’s parents gave to the Hampton family, which had been portrayed as a gift, amounted to severance money and violated federal campaign finance laws. The committee’s counsel, Carol Elder Bruce, alleged that Ensign provided false statements to the Federal Election Commission as it reviewed a complaint about the matter.

Criminal cases against senators are rare and difficult to prosecute.

In the past 30 years, just three sitting or former senators have faced federal felony criminal charges. Harrison “Pete” Williams (D-N.J.) was convicted in 1981 and resigned the next year in the fallout from the Abscam scandal. Charges were dismissed against former senators David Durenberger (R-Minn.), in a 1993 case connected to his personal finances, and Ted Stevens (R-Alaska), who was accused of accepting illegal gifts from a donor.

Legal experts said that the allegations against Ensign are not likely to lead to prison time and that many defendants are put on supervised probation or a few months in jail.

Others pointed to a possible stiffer sentence if it is proved that Ensign intentionally gave false statements to the FEC during its review.

Debra Katz, a Washington lawyer who specializes in discrimination and ethics cases against lawmakers and political appointees, said the Senate’s extensive findings of fact are damning and could expose the former senator to jail time should he be convicted. She said the pattern of his actions suggests an abuse of his office and an attempt to conceal criminal actions.

“This wasn’t either a severance payment or a gift,” she said of the money Ensign’s parents gave the Hamptons. “It sure looks like he paid hush money and then tried to cover it up.”

Since June 2009, when Ensign acknowledged the affair, the ethics committee has conducted an exhaustive investigation, interviewing 72 witnesses, obtaining e-mails and studying his personal journal.

Ensign began an affair with Cynthia Hampton, his political treasurer, in late 2007 after the Hamptons’ home had been burglarized and the family was staying with the Ensigns. The ethics allegations, however, focus entirely on the former senator’s actions after he ended the affair in 2008 and dismissed the couple.

Ensign had made an “extraordinary effort,” the report said, to enlist Nevada corporate executives to hire Douglas Hampton, reaching out to Allegiant Airlines, NV Energy and several other firms.

When prominent Las Vegas developer Paul Steelman declined to hire Hampton, the report said, Ensign told his chief of staff, John Lopez, “to jack him up to high heaven” and make clear to Steelman that he was “cut off” from the senator and should not contact him in the future.

“I really felt like this is wrong. I remember really feeling like that was abusing the office,” the report quoted Lopez as testifying.

In December 2010, another Ensign attorney, Paul Coggins, announced that the Justice Department was no longer investigating Ensign and that federal prosecutors had indicated to him they had “no plans” to charge him.

On Thursday, Coggins said in an e-mail to The Washington Post that he had based his announcement on prosecutors’ statements that Ensign was not a “target” of the investigation at that time, a specific designation for someone who is about to be charged by the Justice Department.

Coggins said Thursday that he stands by his earlier statement. “Now that Senate ethics has made a referral, the Department of Justice will look into it, and I am confident that once again the Department of Justice will conclude that Senator Ensign fully complied with the law,” he said.

Ensign — once considered a rising star who held GOP leadership roles and traveled to Iowa in a brief test of a White House bid — spent his last days in the Senate as an isolated figure. In a farewell address before an empty Senate chamber the day before his resignation, he blamed the affair on his arrogance and urged others to find staff members and friends who would shut down any reckless behavior.

“My caution to all of my colleagues is to surround yourself with people who will be honest with you about how you really are and what you are becoming and then make them promise not to hold back,” he said. “I wish I had done this sooner, but this is one of the hardest lessons that I’ve had to learn.”