"Its effect is to critically undermine the Executive Branch's ability to investigate and prosecute corrupt activity in and affecting the Legislative Branch," Justice Department lawyers wrote in a December 2007 brief. They said that vital investigative techniques could be compromised and that they might be forced to give lawmakers notice before the FBI searched their homes, vehicles or briefcases.
The Supreme Court declined to hear the case, which had the effect of making the restrictions law in the District.
Jefferson was nevertheless convicted of corruption in a case made famous by $90,000 found in his freezer.
Despite decisions to drop cases, Justice Department and FBI officials say they are as aggressive as ever in confronting congressional and other public corruption. They say they are increasingly focused on corruption nationwide and cite an influx of FBI agents to fight corruption along the nation's Southwest border and the recent indictment of 11 people, including four current and former state lawmakers, in a bribery inquiry related to bingo legislation in Alabama.
During the past two weeks, a federal grand jury in Richmond indicted a former member of Virginia's House of Delegates, Phillip A. Hamilton, on bribery and extortion charges, and Paul J. Magliocchetti, a former D.C. lobbyist, was sentenced to 27 months in prison for illegal campaign contributions.
"They may not be the sort of cases that will make headlines in Washington, but I think it's the sort of thing the American public wants us to do," said Jack Smith, who became chief of the Justice Department's Public Integrity Section in May.
A review by Smith and other officials of hundreds of public corruption investigations was a key factor in the decisions to drop some of the inquiries on Capitol Hill. Smith thought some investigations had lagged too long without sufficient progress, officials said.
Watchdog groups have criticized House leaders for their stance on speech or debate. The House brief in the Renzi case "was outrageous," said Melanie Sloan, executive director of Citizens for Responsibility and Ethics in Washington. "They're basically saying that if you are a member of Congress, you cannot be wiretapped under any circumstances."
Irvin Nathan, who resigned this month as House counsel, countered that prosecutors can use evidence collected through wiretaps or other means but must exclude anything related to legislative actions.
Speech or debate "does make life harder for the Justice Department, but that's what the framers intended," Nathan said.
Shortly after the court ruling on the search of Jefferson's office, attorneys for Feeney, under investigation in connection with a golf trip to Scotland financed by Abramoff, filed a challenge before a D.C. federal judge in which they cited the Jefferson decision.
That prompted the D.C. Circuit Court to throw out evidence, sought by a federal grand jury, from Feeney's statements to the House ethics committee.
The speech or debate challenge "was absolutely the reason" the Feeney investigation was dropped in 2009, a source familiar with the probe said, speaking on the condition of anonymity because the information is not public.
Doolittle, under investigation for his ties to Abramoff, also mounted a defense based in part on speech or debate.
Sources familiar with the investigation said Doolittle challenged a search of his Virginia home, along with a Justice Department subpoena for records. The sources said the battle hampered the investigation, but they would not comment further, citing sealed court proceedings.
And several years ago, the House general counsel's office challenged the subpoena of a House appropriations panel staff member in the probe of a lobbying firm's ties to Lewis. Sources said prosecutors wound up interviewing the staffer anyway.
The U.S. attorney's office in Los Angeles said last month that the Lewis investigation had been closed.
Staff writer Carol D. Leonnig and staff researcher Julie Tate contributed to this story.