Sen. Edward W. Brooke's beleaguered bid for a third term received a boost yesterday from top GOP fund-raiser Henry A. Kissinger, who praised the Massachusetts Republican as a "national asset."

Kissinger's two-hour foray into Massachusetts drew over 200 Republicans at $100, $250 and $500 a head to the USS Constitution museum in the historic Charlestown Navy Yard here for conversation and cocktails" with the former secretary of state.

"I strongly endorse and support his candidacy because I think Ed is a national asset." Kissinger told a news conference here. "He is a man of courage, integrity and character who has stood for a strong America and a conciliatory America.

Kissinger, currently a counselor to the Center for Strategic and International Studies at Georgetown University and recently appointed chairman of the board of directors of the North American Soccer League, stressed twice during the news conference that his support for Brooke was voluntary and that the had not been asked by the senator for help.

Kissinger's appearance for Brooke - troubled throughout his campaign by bad publicity over his bitter divorce and more recently by a Senate Ethics Committee probe into his finances - comes at a time when polls here show the senator either behind or neck-and-neck with his liberal Democratic opponent, Rep. Paul Tsongas.

The fundraising effort also escalated the embattled senatory's war of endorsements against Tsongas.

Along with Kissinger, Brooke has received support from conservative Republican Ronald Reagan, who plans to campaign here for him, and feminist Gloria Steinem as well as several nationally known black leaders, including the Rev. Jesse Jackson.

Tsongas has countered with endorsements and aid from President Carter, who plans to make a brief campaign stop here Oct. 28, the president's mother, Miss Lillian, and his nationally known Massachusetts congressional colleagues, Sen. Edward M. Kennedy and House Speaker Thomas P. (Tip) O'Neill Jr.

Tsongas has also drawn support from various Democratic policital figures, including Se. Muriel Humphrey (Minn), Sen. Frank Church (Idaho), Rep. Morris K. Udall (Ariz), Rep. Peter Rodino (N.J.) and Rep. Barbara Mikuiski (Md.), a leader in the drive for the Equal Rights Amendment.

The flurry of national Democratic support for Tsongas was engineered in large part by the Democratic Senate Campaign Committee, which targeted the Masschusetts Senate race for a major effort designed to wrest the seat away from the Republicans for the first time in nearly 50 years.

Brooke's political problems stemming from his divorce have also added to the national enthusiasm for Tsongas' candidacy. "There's a feeling in Washington that Brooke can be had," said his aide, Ed Pinto.

Brooke barely survived in the primary against conservative radio and TV talk show host Avi Nelson, after his funds began to dry up this past summer in the wake of speculation he would quit the campaign.