Tom DeLay resigned as U.S. House majority leader on Wednesday after a Texas grand jury indicted him and two associates on charges of conspiracy in fundraising. A defiant DeLay insisted on his innocence and called the prosecutor a "partisan fanatic."

DeLay is the first leader of the House of Representatives to be indicted while in office in at least a century, according to congressional historians.

Former U.S. House majority leader Tom DeLay (AP Photo/Dennis Cook)

"I have done nothing wrong. ... I am innocent," DeLay said at a news conference during which he criticized the Texas prosecutor, Ronnie Earle, repeatedly, calling him an "unabashed partisan zealot." DeLay said the charges amounted to "one of the weakest and most baseless indictments in American history."

The grand jury accused DeLay, 58, of a conspiracy to violate Texas election law, which prohibits use of corporate donations to advocate the election or defeat of political candidates. The alleged scheme had the donations going to a DeLay-founded political committee, redirecting to the Republican National Committee and eventually to GOP candidates in Texas.

Criminal conspiracy is a state felony punishable by six months to two years in a state jail and a fine of up to $10,000 US.

In responding to accusations that the charges were politically motivated, Earle said, "Our job is to prosecute abuses of power and to bring those abuses to the public."

Republicans expressed their backing for DeLay, and stressed a need to focus on the GOP agenda of immigration, the budget and repair and recovery after two hurricanes.

"He will fight this and we give him our utmost support," said Speaker Dennis Hastert of Illinois following a private GOP meeting.

However, opponents remained on the attack. Democratic chairman Howard Dean cited the problems of DeLay and Karl Rove, the White House deputy chief of staff at the centre of questions about the leak of a CIA operative's name. "The Republican leadership in Washington is now spending more time answering questions about ethical misconduct than doing the people's business," Dean said.

DeLay said he was certain the indictment would be dismissed and shrugged off the charges as a politically motivated effort to drive a wedge in the Republican ranks.

"I have notified the speaker that I will temporarily step aside from my position as majority leader pursuant to rules of the House Republican Conference and the actions of the Travis County district attorney today," DeLay said in a statement.

Following the resignation, Republicans in the U.S. House of Representatives unanimously made party whip Roy Blunt of Missouri their acting majority leader

Lawmakers said Blunt would share leadership responsibilities with Rep. David Dreier of California.