The biggest scandal involving a sitting U.S. senator in 16 years has resulted in a bombshell: the Senate ethics committee believes Republican John Ensign committed crimes, and has referred the matter to the Justice Department for prosecution.

Senate Ethics Committee special counsel found “substantial credible evidence that provides substantial cause to conclude that [former Sen. John] Ensign violated Senate Rules and federal civil and criminal laws, and engaged in improper conduct reflecting upon the Senate, thus betraying the public trust and bringing discredit to the Senate,” according to a report posted to the committee website.

The Ethics Committee referred the evidence of possible of possible legal violations to the Justice Department on Thursday.

The findings come nearly two years after the ethics committee began its probe, and nine days after Ensign resigned from the Senate in disgrace. He was scheduled to speak to the committee under oath on May 4, prompting his departure on May 3.

Specifically, in its letter (pdf) to Attorney General Eric Holder, the ethics committee’s members presented evidentiary materials showing Ensign “aided and abetted violations of the one-year post-employment contract restriction,” “conspired to violate that restriction,” “made false statements to the Federal Election Commission,” “violated campaign finance laws,” and “obstructed the Committee’s preliminary inquiry.”

Sen. Barbara Boxer (D-Calif.) told reporters that Ensign, had he stayed in the Senate, would now be facing an expulsion vote.

For those of us who’ve followed this scandal closely, this isn’t at all a surprise. Ensign has looked really guilty for a very long time.

To briefly recap for those who’ve forgotten — it’s always irked me that major media outlets almost completely ignored this story — Ensign’s humiliation came to public attention in June 2009, when we learned the conservative, “family-values” senator carried on a lengthy extra-marital relationship with one of his aides, who happened to be married to another one of his aides. Ensign’s parents tried to pay off the mistress’ family.

The scandal grew far worse when we learned that the GOP senator pushed his political and corporate allies to give lobbying contracts to his mistress’s husband. When Douglas and Cynthia Hampton left Ensign’s employ — because, you know, the senator was sleeping with Cynthia — Ensign allegedly took steps to help them make up the lost income, leaning on corporate associates to hire Douglas as a lobbyist, and ignoring ethics laws that restrict how quickly former aides can begin lobbying careers.

Maybe now the major media outlets who’ve ignored this story for years — I’m looking in your direction, Washington Post — will realize this is a big deal? I mean, really. How often does a senator get busted this severely in a scandal with such a broad scope?

I’m obviously aware of the IOKIYAR rule, but this is a major development.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.