"The essence of speech or debate is you can't introduce into evidence what the member did if it's part of an official duty," said a lawyer who spoke on the condition of anonymity because the challenges are often under court seal. "If you can't introduce legislation, a bill, a speech on the floor, how do you make the case?''
In the Visclosky investigation, prosecutors explored whether the congressman helped obtain earmarks for clients of powerful lobbying firms in exchange for contributions. But Visclosky's attorneys refused to hand over much of the material sought in 2009 by a federal grand jury in Alexandria, citing speech-or-debate concerns, according to sources familiar with the investigation.
The investigation has not formally ended, but the sources said Visclosky is unlikely to be charged. Justice Department officials declined to comment.
The dropped investigations covered subjects ranging from Ensign's alleged efforts to keep secret his affair with a staff member to a $10 million earmark backed by Young for a Florida road project.
Mollohan announced last year that he will not be charged in a probe of his finances and nonprofit groups he created and helped fund in his district.
In an interview, Mollohan said the investigation was politically motivated. In each of the other cases, either the exonerated lawmaker or his attorney has disputed the allegations.
"I think it's a bum rap for people who don't know the evidence to assume there is something improper when career prosecutors determine they're not going to indict somebody," said Richard Cullen, a former U.S. attorney in Virginia who is chairman of McGuireWoods in Richmond. He represents DeLay, who announced in August that the Justice Department had dropped its six-year probe of his ties to disgraced lobbyist Jack Abramoff.
Sources familiar with the federal investigation said that DeLay did not hold back documents from prosecutors but that they were hampered by their inability to persuade a key witness to testify against him.
Attorneys for Renzi, whose case has been delayed, have cited the speech-or-debate issue in their argument that the government violated his rights when agents wiretapped his cellphone, interviewed congressional aides without his consent and procured documents that aides took from his office.
Renzi's attorneys received support in June from the House general counsel, who argued in a brief on behalf of House leaders including John A. Boehner (R-Ohio) and Nancy Pelosi (D-Calif.) that the Justice Department "repeatedly and flagrantly violated" Renzi's constitutional rights.