The government-besieged Teamsters union is celebrating an end to its 32-year marriage of convenience with Republican presidential candidates, and has chosen to dance with a new partner at the Democratic National Convention. Even though the union's new president is an Irish Catholic Bostonian who supported Michael Dukakis' opponents in previous elections, the Teamsters hosted a party here Sunday night symbolizing a return to the labor movement's half-century courtship with Democrats. William J. McCarthy's upset election last week to succeed the late Jackie Presser as the Teamsters' president had cast doubts among some union leaders that organized labor could unite behind the Democratic presidential nominee for the first time since 1956. McCarthy, 69, had supported Edward King in both his 1978 upset of the incumbent Dukakis for the Democratic gubernatorial nomination in Massachusetts and had opposed Dukakis' successful challenge of King four years later. ``That sort of personalizes it,'' one official of another union commented. McCarthy is known among other labor leaders to have opposed the Teamsters' reaffiliation with the AFL-CIO last October. Implicit in the reunion was an understanding that the Teamsters would go along with the labor federation's Democratic presidential endorsement. Commanding a fourth of the delegates at this week's convention, labor leaders have openly wondered whether the new Teamsters chief would abide by Presser's promise to work with the rest of labor ``to create the biggest political giant this country has ever seen.'' With the largest political campaign chest of any group in the country, the question is not academic. At the end of March, the Teamsters' DRIVE (Democratic, Republican, Independent Voter Education) political action committee had $5.5 million _ $1.5 million more than the American Medical Association and $1.8 million more than the National Association of Realtors. Teamster political operatives sought to assure more than a dozen presidents of other unions here Sunday that McCarthy's upset election last Friday over Dukakis supporter Weldon Mathis, Presser's designated heir, will not return the union to the GOP camp. ``There's about as much chance of us endorsing George Bush as there is it snowing in Atlanta tomorrow,'' said one senior Teamster official. The Teamsters generally had courted Republican presidential candidates ever since Dwight Eisenhower's bid for re-election in 1956. Presser had been the only major labor leader in the country to support Ronald Reagan in the 1980 and 1984 elections, saying then the AFL-CIO was out of touch with workers in its endorsements of Democrats Jimmy Carter and Walter Mondale. But Presser, who died July 9, began grumbling about the Reagan administration more the four years ago, first over its appointees to the National Labor Relations Board and then over his indictment in 1986 on labor racketeering and embezzlement charges. The final straw, according to Teamster officials, was the government's suit to wrest control of the union from its leaders on a claim they are puppets of organized crime. McCarthy was notably absent from Sunday's reception. Teamsters and other labor leaders said that is characteristic of the low priority he places on national presidential politics. Upon taking over the union's reins last Friday, McCarthy said he would have no comment on a Teamsters presidential endorsement ``until after both conventions, Democratic and Republican'' _ the same position taken officially by the AFL-CIO. But a Teamster official noted that, despite his endorsement of Dukakis' opponent in Massachusetts gubernatorial races, McCarthy had served on the governor's labor-management council. And a senior AFL-CIO official acknowledged that the Teamsters were not alone in abandoning Dukakis at the end of his first time as Massachusetts' governor. ``About half the labor movement was against him then,'' the official said, speaking on the condition of anonymity. ``He messed up his first term.'' Another Teamster official also noted that Dukakis _ although he was the last of the Democratic presidential candidates to do so _ had spoken against a Justice Department suit to take over the 1.6-million-member union on a claim its leaders are puppets of organized crime. ``George Bush is the only candidate out there now who has not spoken against the suit,'' said Tim O'Neill, a spokesman for the union. Asked if the Teamsters planned a similar reception at the GOP convention next month in New Orleans, another official commented coldly: ``We see no reason to be there.''