A constitutional clause designed to protect members of Congress from abusive or harassing lawsuits is increasingly being used by lawmakers as a shield in public corruption investigations, frustrating investigators even as the FBI attempts to police wrongdoing at a pace not seen since the Watergate scandal.

Under a constitutional provision known as the "speech or debate clause," lawmakers have wide protections that cover their work on Capitol Hill. That means legislation, floor speeches, and wiretaps that capture information related to votes and strategy are often out of bounds in developing a criminal case.

The latest lawmaker to seize on the controversial legal argument is Rick Renzi (R-Arizona), who is citing the wiretaps of his Verizon Wireless BlackBerry in trying to persuade a court to throw out charges of fraud, extortion and conspiracy against him.

For four weeks surrounding the 2006 midterm elections, FBI agents secretly listened as Renzi and fellow House members traded phone calls to gossip about congressional leadership races and fret over the future of the Republican Party. The conversations also revealed intrigue and favor-trading among House members and their aides.

Earlier this week, Renzi received a boost when the House leadership, both Republicans and Democrats, asked the judge in his case for permission to file a friend-of-the-court brief in support of at least some of Renzi's arguments.