WHAT PUSHED ENSIGN OUT THE DOOR…. As recently as a few months ago, Sen. John Ensign (R-Nev.) thought he’d come up with a smart strategy: ignore his scandal and pretend like everything’s normal. Despite the criminal probe, ethics investigations, and humiliating sex scandal, the conservative Republican senator, as of February, even planned to seek re-election.

The tide turned fairly quickly. In March, Ensign announced he wouldn’t seek another term, but assured the public he would stay in office through 2012. This week, Ensign switched gears again, and announced his resignation, effective May 3.

Why did Ensign reverse course and step down in disgrace? Probably because of what would have happened on May 4.

Senator John Ensign’s resignation letter allows him to leave office just one day before he was to have to answer questions under oath about whether a $96,000 payment to the family of his former lover was illegal and designed to keep the affair from becoming public, according to people familiar with an investigation of Mr. Ensign’s activities.

That formal testimony, scheduled for May 4, was the final step as Senate investigators prepared for what were almost certain to be Senate ethics charges against Mr. Ensign, Republican of Nevada. Mr. Ensign’s resignation is effective May 3.

In the letter, issued late Thursday, Mr. Ensign acknowledged he was stepping down to avoid further scrutiny — hoping that his departure from the Senate would mean the end of any further questions about his affair with Cynthia Hampton, the wife of his former senior aide, Douglas Hampton.

When the Republican senator announced in March he’d retire at the end of his term, he said he wouldn’t resign for a simple reason: it would give the appearance of guilt. Now Ensign is resigning, the day before he was scheduled to testify under oath.

What was that about appearances?

Keep in mind, Ensign’s departure does not let him off the hook entirely. Once he’s gone, the Senate will no longer have the power to punish him — he’ll no longer be a member. But the Senate ethics committee, which has been engaged in a 22-month investigation, may yet release the findings of its probe, and if the committee found evidence of criminal wrongdoing, it may also refer the evidence to the Justice Department for prosecution.

We can’t say with any confidence what the ethics committee has on Ensign, but it’s worth emphasizing that the chair and vice chair of the panel issued a statement late Thursday that read in part, “The Senate Ethics Committee has worked diligently for 22 months on this matter and will complete its work in a timely fashion. Senator Ensign has made the appropriate decision.”

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.