Former representative Daniel J. Flood (D-Pa.), whose long and powerful tenure in Congress collapsed under criminal charges that he traded his political influence for cash, pleaded guilty in U.S. District Court yesterday to conspiracy to solicit illegal campaign contributions and was immediately placed on one year of probation.

In exchange for the guilty plea to a misdemeanor charge, government prosecutors dismissed more serious conspiracy, bribery and perjury charges against Flood, 76, who repeated yesterday that he was innocent of any wrongdoing.

"I want my friends, the people of the 11th District of Pennsylvania and elsewhere, to know that just as I have done from the very start, I deny with all my heart that I have committed any criminal offense," Flood said in a prepared statement that he read to reporters outside the courthouse after the proceeding.

"The terrible burden of this case has been damaging to my health and I need peace from it," said Flood, who has been hospitalized for a variety of illnesses, including drug dependence, over the past several months.

Judge Oliver Gasch accepted Flood's plea under the terms of a 1970 U.S. Supreme Court decision which permits a defendant to enter a guilty plea while maintaining innocence, because of the overwhelming weight of the government's evidence and the likelihood of conviction at trial.

The once-flamboyant Flood, a lawyer and former stage actor who now appears frail and drawn, was scheduled to stand trial, a second time before Gasch on April 8 on the conspiracy, bribery and perjury charges. The government had alleged that Flood received more than $50,000 worth of bribes from persons who wanted his help in obtaining government favors.

Flood's first trial ended with a hung jury in February 1979.

Flood, who was chairman of a House Appropriations subcommittee, resigned from Congress Jan. 31 of this year, in the midst of his 16th term. Only two House members had come to Congress earlier.

The government's chief prosecutor in the Flood case, Mark H. Tuohey III, said yesterday that Attorney General Benjamin R. Civiletti and other Justice Department officials were "fully satisfied" with the plea agreement, reached in a secret meeting in Gasch's chambers last Friday. Justice Department Attorney Bernard J. Panetta II also participated in the case.

Tuohey, who said Flood's age and poor health were factors in the government's decision, said that the department has vigorously litigated every aspect of the case against Flood, both at trial and at lenghty competency hearings before Gasch in January.

At those hearings, lawyers, Axel Kleiboemer and Walter H. Fleischer tried unsuccessfully to persuade Gasch that Flood's mental capacities were so deteriorated that he could not assist his lawyers in preparing for a second trial.

Yesterday Flood, dressed in a neat three-piece brown tweed suit, told Gasch that he was pleading guilty to the conspiracy charge because he had concluded that "I might be convicted in a trial" on the previous charges. Flood added, "I do not think that I have the physical or intellectual resources necessary to defend myself adequately."

Specifically, Flood pleaded guilty to defrauding the United States by committing a violation of the Federal Election Campaign Act, a misdemeanor punishable by a maxinum of a year in jail or a $25,000 fine.

The conspiracy charge said that, between September 1970 and June 1976, Flood and his administrative assistant, Stephen B. Elko, illegally solicited money and stock in the form of campaign contributions from various persons seeking contracts with the federal government. Elko was the government's key prosecution witness.

Between 1971 and 1973, the charge said, Flood solicited money from Dr. Murdock Head, director of the Virginia-based Airlie House Foundation, who was seeking contracts from the Department of Health, Education and Welfare and the Agency for International Department. Elko, in Flood's behalf, received $2,500 from Head, the charge said.

Flood also pleaded guilty to taking 100 shares of stock from a Pennsylvania bank trustee who sought federal approval of a bank merger. In October and November 1974, the charge said, Flood solicited a campaign contribution from a home builder who wanted federal housing subsidies for an apartment project. As a result, Elko received $3,000 which was delivered to Flood's Pennsylvania campaign account.

Finally, the government said, Flood in 1975 and 1976 sought campaign contributions from a New York rabbi who wanted federal help for a manpower project. Elko received checks totaling $1,000 from the rabbi for the Flood campaign account and Flood himself received a $1,000 check, the charge said.