WASHINGTON — Senator John Ensign’s top aide warned him in 2008 that his office might be breaking the law by helping a former staff member build up a lobbying business. The senator let out a long groan, then said they should help the former employee anyway, according to a remarkably detailed Senate report that urged the government to consider criminal charges against Mr. Ensign.

The report issued Thursday, a result of the most extensive Senate ethics investigation in at least two decades, describes the actions inside the senator’s office as he sought to manage the fallout from an affair with the wife of one of his former senior aides.

Perhaps seeking to cover up the wrongdoing, staff members deleted incriminating e-mails, and the senator himself may have lied under oath to thwart a separate investigation by the Federal Election Commission, according to the bipartisan report.

Mr. Ensign’s actions were so brazen and improper that had he not resigned last week he might have been the first senator expelled in nearly 150 years, said Senator Barbara Boxer, a California Democrat who is the Ethics Committee chairwoman.