The union representing most government tax collectors, TV and radio regulators and federal election monitors took a big political plunge yesterday: It:

Endorsed Sen. Edward M. Kennedy for the presidency.

Donated $5,000 from a special warchest of the NTEU (National Treasury Employes Union) to the Kennedy presidential campaign.

And blasted the boss, Jimmy Carter, who has at least 15 months left as president, saying he is "totally unacceptable" to the majority of the federal workforce.

NTEU is the government's third largest union.

The endorsement on behalf of the 65,000 members was made by its president, Vincent L. Connery. This is the first political endorsement of the season from any federal or postal unions. Most of those organizations, who represent more than half the 2.7 million federal workers, are expected to wait much longer before jumping on, or off, any political bandwagon.

Top Kennedy staffers were on hand for the Capitol Hill news conference called by the independent union.Rep. Fortney (Pete) Stark, the California Democrat who arranged for the room, welcomed Connery with a whispered "it's nice to be out of the closet and get to work" when Connery came in. Stark was one of the first congressional Democrats to bolt Carter and urge Kennedy to run.

NTEU represents about 115,000 workers. They are strategically located in the Treasury Department, with a special stronghold in the Internal Revenue Service. NTEU also bargains for and/or represents workers at the Federal Elections Commission, HEW, the Federal Communications Commission and Department of Energy. Connery said he will call on other federal union leaders to join him in a coalition to back Kennedy. In 1976 most federal and postal unions (including the NTEU) worked to dump incumbent President Gerald Ford in favor of candidate Jimmy Carter.

The union leader said Carter has not lived up to promises he made federal workers in 1976. He said Carter reduced the size of Catchup-with-industry raises due federal workers. He was especially bitter that Carter has "ridiculed" civil servants, particularly when selling Congress and the public his civil service reform plan. In response to a question Connery said he had "no promises" on pay from Kennedy, but he said the Senator's record of "devotion" to working people is the best hope of civil servants.

In addition to the natural reluctance to get trampled by a premature political stampede, most federal and postal unions have constitutions that prevent their leaders from endorsing candidates on behalf of the union. An NTEU spokesman said the independent union had no such bar.

Federal workers, Connery said, "have been excoriated by Jimmy Carter more than by any political figure in recent history. He [Carter] has denigrated and cast aspersions on federal employes at every opportunity."

Connery accused Carter of cutting back federal pay raises (5.5 percent in 1978 and 7 percent this year) "to exploit to his advantage the public's dissatisfaction with big government." Carter's new civil service reform act, Connery said, "threatens to politicize" the middle and top-management ranks of the career civil service. Despite his promise to be honest and fair, Connery said, the president had "utterly failed to treat federal employes with honesty and fairness . . ."

Kennedy press aide Tom Southwick said he could make no promises about future federal pay raises or fringe benefits. He predicted that Kennedy, as president, would not put a lid on federal wages unless pay or private workers, and prices, also were controlled.

Southwick said Kennedy is grateful to the NTEU and promised that "one year, two years from now we will not forget who was with us early" in the campaign. Kennedy's formal announcement is expected Nov. 7.