Sen. Edward W. Brooke (R-Mass.), plagued by controversy over his personal life throughout his reelection campaign, apparently was defeated in his bid for a third term yesterday by liberal Democratic Rep. Paul E. Tsongas.

CBS News projected Tsongas a decisive winner over Brooke, the only black in the Senate.

A Senate seat held by another liberal Republican Clifford Case of New Jersey, who lost in the primary, was also apparently falling to a liberal Democrat, former professional basket-ball star Bill Bradley, who led in his race against conservative Republican Jeffrey Bell.

The GOP appeared headed for an important victory in Pennsylvania where underdog Richard Thornburgh, a former assistant U.S. attorney general, was leading former Pittsburgh Mayor Pete Flaherty for governor.

Republican also had hopes of another upset in New Hampshire, where early returns showed Gordon Humphrey, a 37-year-old airline pilot and conservative activist, leading incumbent Democratic Sen. Thomas J. McIntyre.

Elsewhere in the East, most incumbents in statewide races appeared to be doing well from early returns.

According to television network projections, Democratic Govs. Hugh Carey of New York and Ella T. Grasso of Connecticut won reelection, as did Republican Govs. Meldrim Thomson of New Hampshire and Richard A. Snelling of Vermont.

The networks also declared as winners incumbent Sens. Joseph R. Biden Jr. (D-Del) and Claiborne Pell (D-R.I.) and Democratic Gov. J. Joseph Garrahy of Rhode Island.

While the Democratic appeared to have picked up a Senate seat with the Massachusetts defeat of Brooke, they were in danger of losing a seat in Maine. CBS News was projecting Republican Rep. William S. Cohan the winner over incumbent Democratic Sen. William D. Hathaway.

In Maryland, former transportation secretary Harry Hughes, a Democrat, held a commanding lead in early returns for governor over former Republican Sen. J. Glenn Beall. Connecticut

Democratic Gov. Ella T. Grasso held an early lead and appeared headed for reelection in Connecticut last night over Republican Rep. Ronald A. Sarasin.

Grasso, 59, who had faced a strong primary challenge, waged an anti-income tax campaign to maintain her record of never having lost an election in 26 years in politics. Delaware

U.S. Sen. Joe Biden (D-Del) won a second term yesterday, according to ABC news projections.

Biden, 35, defeated former Sussex County Republican chairman James H. Baxter Jr., 54, who was offered by his party as something of a sacrificial candidate.

Other well-known Republicans in Delaware had turned down requests to run against Biden, who had adopted a tough antibusing stand in a state that recently saw the implementation of one of the largest court-ordered school busing plans to go into effect this year. New Jersey

Former professional basketball star Bill Bradley won his race for a Senate seat from New Jersey, according to projections last evening by CBS and NBC television.

The 35-year-old Democrat, making his first political race, was the favorite throughout the campaign against Republican Jeffrey Bell, a conservative who campaigned heavily on taxcut issues and had defeated Sen. Clifford Case (R.-N.J.) in the primary election. Pennsylvania

Republican Richard L. Thornburgh upset Democratic favorite Pete Flaherty in the Pensylvania governship race, according to CBS projections.

In Philadelphia, charges of widespread voting irregularities marred the electoral referedum on a third term for controversial Mayor Frank Rizzo.

Flaherty, former mayor of Pittsburgh, had ben heavily favored to win, in part on the strength of the Democrats' statewide registration lead of 850,000 voters.

Thornburgh was head of the Justice Department's criminal division during the Ford administration and initiated criminal investigations centering on the administration of outgoing governor Milton Shapp.

The vote on the amendment to change the Philadelphia city charter, allowing Rizzo to seek a third term, was clouded by numerous reports of voting irregularities and machine jamming. The FBI was called to investigate.

One city voting commissioner, Margaret Tartaglione, a staunch Rizzo ally, was arrested on charges that she improperly changed a polling place and thus denied hundreds of voters a chance to vote on the amendment.

Rizzo, who had urged voters to "vote white," reportedly was getting strong endorsement in South Philadelphia, a white ethnic stronghold.

But in West Philadelphia and other predominantly black areas of the city where he was expected to fare poorly, voting was hampered by up to 400 voting machine breakdowns and other irregularities.

Anthony Jackson, an attorney for the city's Black United Front organization, won a last-minute court order to have up to 20,000 paper ballots printed and circulated in the areas where the machines broke down.

Jackson told reporters he believed the difficulties "were absolutely intentional - an attempt by Mayor Rizzo's supporters to prevent blacks from voting."

Neither Rizzo nor his aides could be reached immediately for comment.

Assistant U.S. Attorney Pete Vaira called for an investigation into the irregularities. And, at last report, FBI agents had served subpoenas on city voting officials to answer questions about the numerous machine mal-functions. Vermont

In first fragmentary returns from Vermont, incumbent Republican Gov. Richard A. Snelling held a strong lead over his Democratic challenger, Edwin Granai, majority whip of the Vermont House of Representatives.

The same early returns showed the state's lone member of Congress, Republican Rep. James M. Jeffords, winning an easy victory over Democrat S. Marie Dietz, an anti-abortion activist.