WASHINGTON – Eyeing an opportunity to win back sorely missed congressional seats, Democrats will cast yesterday’s indictment of a top Republican in the House of Representatives and the opening of a securities investigation into trading by the Republican majority leader of the Senate as symptoms of widespread corruption within a political party that controls the presidency and both houses of Congress.

A Texas grand jury yesterday indicted the Republican majority leader of the House, Rep. Thomas DeLay, of Texas, and two of his political associates on one count each of conspiracy to commit campaign finance fraud. Mr. DeLay, who had been under investigation for a year, declared his innocence at a press conference with reporters and described the indictment as a witch hunt orchestrated by his political enemies on the left.

“As for the charges, I have the facts, the laws, and the truth on my side,” a defiant Mr. DeLay said, “just as I have against every other false allegation my opponents have flung at me over the last 10 years.”

It was the second time in a day that a leader of the Republican Party had been tarnished by a cloud of wrongdoing. The U.S. Securities and Exchange Commission yesterday opened a formal investigation into a stock sale by Senator Frist, of Tennessee, according to a Bloomberg News report. Dr. Frist, a potential candidate for president in 2008, sold shares in the hospital operating company founded by his father, HCA Incorporated, one month before a slide in the stock’s value. He has not been formally accused of illegal trading.

Democrats drew a connection between the indictment, the investigation of Dr. Frist, and other ethical questions surrounding Republicans, drafting an attack strategy that is likely to find its way into Democratic campaign platforms leading up to the midterm elections next year. That strategy was neatly summed up by the minority leader of the House, Rep. Nancy Pelosi, of California, who called Mr. DeLay’s indictment “just the latest example that Republicans in Congress are plagued by a culture of corruption.”

The chairman of the Democratic National Committee, Howard Dean, predicted Republican difficulties were just beginning.

“Today, the state of Texas is doing what the Republican-controlled federal government has failed repeatedly to do,” Dr. Dean said in a statement, “which is to hold Republicans in Washington accountable for their culture of corruption. This alleged illegal activity reaches to the highest levels of the Republican Party…. Tom DeLay is neither the beginning nor the end of the Washington Republicans’ ethical problems.”

The indictment against Mr. DeLay is connected to a plan he hatched in 2001 to help Republicans gain control of the Texas House the following year.A political-action committee that Mr. DeLay formed, Texans for a Republican Majority, was indicted earlier this month for accepting contributions from corporate donors to legislative races. Corporations are not permitted to contribute to candidates under Texas law.

House rules require the 58-year-old Mr. DeLay, nicknamed “The Hammer” for the hard-line tactics he is said to use in keeping party members in line and on message, to temporarily resign from his leadership post. He will be replaced by Rep. Roy Blunt, of Missouri, as majority leader of the House.

The indictment follows an investigation by a district attorney from the Austin area, Ronnie Earle, who Republicans have long accused of being a Democratic political pawn. Mr. Earle brought charges that were later dropped against Senator Hutchison, a Republican of Texas, in 1993, for using state equipment for a campaign. Mr. Earle has defended himself against accusations of political favoritism in the past by saying that he has indicted more Democratic politicians than Republicans in his career.

Republicans who defended Mr. DeLay yesterday repeated the charge against Mr. Earle, with the chairman of the National Republican Congressional Committee, Rep. Tom Reynolds of New York, calling Mr. Earle “an unapologetic Democrat partisan.” Mr. Reynolds and other Republicans urged patience. “Until Majority Leader Tom DeLay has his day in court, it is vitally important he be afforded the same presumption of innocence afforded to every other American,” Mr. Reynolds said.

The White House spokesman, Scott McClellan, also urged patience. Calling Mr. DeLay “a good ally,” he told reporters at the White House yesterday that Mr. Bush is reserving judgment on his fellow Texan until after the outcome of the trial. “I think that the president’s view is that we need to let the legal process work,” Mr. McClellan said. “There is a process in place, and we’ll let that work.”

Republicans conceded that Democrats would attempt to capitalize on Mr. DeLay’s indictment.

“I don’t have any doubt that the other side will try to exploit it,” Senator Cornyn, a Republican of Texas, told reporters yesterday. “But I think I’ll just leave it at that.”

Democrats did not devise their assault on Republican ethics yesterday. The party has hinted for weeks that corruption would be a centerpiece of its efforts to gain back the seats they have been steadily losing in both houses of Congress in recent years.

Just hours before yesterday’s indictment, the Democratic minority leader of the Senate, Senator Reid, of Nevada, issued a press release titled “The Bush Administration: A Culture of Cronyism and Corruption.” It detailed alleged improprieties relating to the recovery and rebuilding effort connected with Hurricane Katrina.

“Obviously, we need to allow the process to move forward,” a spokesman for Mr. Reid, James Manley, said later in the day. “But regardless of the outcome, we are desperately in need of reform in this country. And the fact that one of our Republican leaders is under indictment just reinforces a lack of confidence in our government. Recent events demonstrate a need for wholesale government reform.”

Rep. Jerrold Nadler, a Democrat of New York, said the investigation into Mr. Frist’s private sale of family-owned stock should not be viewed as stemming from a corrupt political culture. But, he said, the cloud of wrongdoing that has now settled over both Dr. Frist and Dr. DeLay cannot be good for the Republican Party looking forward.

“It’s a very corrupt political operation, and more and more it’s beginning to show,” Mr. Nadler said. “It’s another indication of just how corrupt the Republican leadership is right now. The allegations against Frist have nothing to do with that. But, having said that, I certainly think there is a political impact of there is a cloud over both majority leaders. Obviously, if there are a lot of clouds over political party, it doesn’t help them.”

Democrats in the House and the Senate announced late yesterday that they will hold a press conference today on an alleged “culture of corruption” within the Republican Party that they say has kept aid from reaching victims of Hurricane Katrina.