Six years after he left the Democratic Party to support Richard M. Nixon, and four years after a jury acquitted him of the charge that he took $10,000 in bribes to boost dairy price supports, former Texas governor John B. Connally did what people have been expecting him to do for the last two decades.

The siver-haired, 61-year-old Houston lawyer yesterday declared his candidacy for president of the United States.

Connally's National Press Club announcement address was a display of the same self-confidence and swashbuckling style that has been his trademark since he left the University of Texas campus 40 years ago this month and came to Washington as secretary to then-representative Lyndon B. Johnson.

Brushing aside President Carter as lacking the perspective and personal qualities for "effective leadership," Connally offered himself to the nation and his adopted party as the man who could be, for "the fourth major period of crisis in our history," what Washington, Lincoln and Franklin D. Roosevelt were for theirs:

"Someone in charge who knows what he is doing and why."

In becoming the fourth declared contender in a Republican field that promises to balloon to 10 or more, Connally staked out a clear conservative position from which to challenge Ronald Reagan, the man he happily conceded is the front-runner for the nomination.

The former treasury secretary outlined a platform supporting a constitutional amendment for a balanced budget, incentives for energy production, eased environmental controls and more leeway for the FBI and CIA.

He said he would not have cut U.S. ties to Taiwan for the normalization of relations with China, and opposed any arms control agreement with the Soviet Union that "freezes the United States into an inferior position."

Connally's characteristic bravado showed most clearly in his answers to the predictable questions on his bribery trial and his links to Nixon.

Asked if he had any regrets about his association with the former president, for whom he served 16 months in the Treasury Department and briefly as a White House adviser, he said: "No, I do not. My relations with him were decent an honorable. I served not him, but the nation. I have no apologies to make for serving in the Nixon administration."

As for the 1975 trial that resulted in his acquittal on charges that he accepted two $5,000 payoffs from dairy producers for supporting a price increase during his tenure in the Nixon Cabinet, Connally said:

"I have faity in the American people, and I think they have faith in the judicial system. I was tried before a Washington, D.C., jury of 10 blacks and two whites... and they gave the answer, I hope for all time... and that answer was not guilty."

A far more caustic appraisal came from Sen. George McGovern (D-S.D.), still steaming more than six years after Connally savaged McGovern's 1972 presidential bid in a television broadside as chairman of the "Democrats for Nixon."

"I wouldn't trust Connally within a mile of the White House," McGovern said in a statement anticipating the formal announcement. "John Connally combines the worst of both Watergate and Vietnam. He's the perfect symbol of the double-talking, double-crossing politician. He doesn't even know what party he belongs to.

"The fact that Connally never went to jail along with the rest of the Watergate gang in positive proof that Ed Williams [attorney Edward Bennett Williams] is the best criminal lawyer in the country."

Asked about the McGovern statement, Connally drew a laugh from the press club crowd by saying, "I gather he's not going to lead the Democrats for Connally in 1980. I'd say he's still a mad McGovern."

Republican officials were more cordial than McGovern to the Connally candidacy, but, privately, many of them also expressed skepticism about his ability to overcome the twin burdens of having been both a lifelong Democrat and a prominent figure in the Nixon era.

But, to a man, they said the force of Connally's personality -- both in person and on the television screen -- could not be underestimated. "I can see him getting 80 percent of the vote in any primary he enters," said one GOP campaign consultant, "or 5 percent."

Connally said he would run in as many primaries as his finances allow, and confirmed that Eddie Mahe, a political professional who had served as executive director of the Republican National Committee, will be the managing director of his campaign, working out of a national headquarters in Arlington.

But the names on the initial list of supporters announced yesterday indicated that Connally had been forced to reach well back in Republican history to find backing.

The group was headed by former postmaster general Winton M. (Red) Blount, a Nixon Cabinet colleague, retiring Sen. Henry L. Bellmon (R-Okla.), Herbert Brownell Jr., Thomas E. Dewey's campaign manager and Dwight D. Eisenhower's attorney general, Eisenhower-era Republican National Chairman Leonard W. Hall, former senator Clifford P. Hansen (R-Wyo.) and Adm. Thomas Moorer, former chairman of the Joint Chiefs of Staff.

Connally was a Johnson political protege for more than 30 years, managing his 1960 presidential bid and serving two years as secretary of the navy in the Kennedy-Johnson administration.

It was as governor of Texas, from 1963 to 1969, that Connally displayed his conservative political instincts. From that point on, he has figured in presidential speculation, and many in the Nixon White House believed he would have had Nixon's support for the 1976 GOP nomination had Nixon not been forced to yield office to Gerlad. R. Ford.

In 1976, Connally hesitated with his endorsement until late, finally going with Ford over Reagan. But he was passed over for the vice presidential nomination, and left the Kansas City convention barely bothering to disguise his frustration.

As he said, only half-joking, yesterday: "There's something I like about inaugurations, but I'm getting tired of being nothing more than a spectator."

Preceding Connally into the GOP race were Rep. Philip M. Crane (R-Ill.), Harold E. Stassen and Los Angeles businessman Benjamin Fernandez.