President Bush vetoed a revision of the Hatch Act yesterday, saying it was necessary to prohibit federal workers from actively participating in partisan politics "to preserve the impartial, evenhanded conduct of government business."

It was Bush's 12th veto and was accompanied by a renewed promise to veto a 13th bill: the family leave measure passed Thursday that would grant workers the right to take up to 12 weeks of unpaid job leave to care for ill relatives and newborn children.

The Hatch Act revision, which passed the House 334 to 87 Tuesday, is expected to trigger a major veto battle in the Senate, where it passed with the minimum 67 votes needed to override a veto.

The Hatch Act was passed in 1939 to prevent federal employees from being coerced to work in the reelection campaign of Franklin D. Roosevelt. It bars federal workers from engaging in most partisan political activities.

The bill would make it possible for federal workers to participate in partisan politics while off duty, to solicit funds from fellow union members and to canvass neighbors door to door for political candidates. It continues to bar civil servants from running for partisan office and from soliciting funds from the public.

Bush said yesterday that Congress had unacceptably altered the "manifestly successful" law, saying that the "obvious result of enactment of {the bill} would be unstated but enormous pressure to participate in partisan political activity."

Although the act contains criminal penalties for coercion of federal workers, Bush said that "subtle forms of coercion are almost impossible to regulate, especially when they arise in a climate in which the unspoken assumption is that political conformity is the route to achievement and security."

Principal supporters of the revision have been federal and postal unions. The opposition has been almost entirely from Republicans, who repeatedly have cited the millions of dollars in contributions made by the unions to Democratic candidates.

Constance Newman, director of the Office of Personnel Management, said that Bush's opposition to the measure, however, is based on "conviction, not politics."

She said that the president's years in government had convinced him of the value of the Hatch Act, which he said shielded civil servants and the programs they administer from political exploitation and abuse.

"I think we have the votes to win" a veto fight, Newman said.

Although Bush is vetoing bills at four times the rate of President Jimmy Carter and twice that of President Ronald Reagan, he has yet to lose a veto battle.

The National Treasury Employees Union asked senators not to "trade their collective conscience for White House cuff links or breakfast with Bush."

Democratic staff members, however, were pessimistic. "I can't believe there isn't a bridge or a judgeship out there that he can use" to pick up the votes he needs to sustain the veto, said one.

In a briefing yesterday, White House press secretary Marlin Fitzwater reiterated Bush's intention to veto the family leave bill. On Capitol Hill, House and Senate Democrats sharply criticized Bush for promising to veto legislation that fulfills a promise he made in his own campaign for the presidency.

"It's time for the president to live up to his words," said Senate Majority Leader George J. Mitchell (D-Maine) at a news conference with Senate and House Democrats.

"The opponents of this bill are Rip Van Winkles yearning for an earlier sexist era when a woman's place was in the home," Sen. Edward M. Kennedy (D-Mass.) said in a statement.

"Earlier this week," said Rep. Pat Schroeder (D-Colo.), "in response to a question about the pending flag amendment, House Minority Leader Robert Michel said that you can't vote against motherhood. Well, George Bush is about to show them how."

Rep. David E. Bonior (D-Mich.) and Kennedy both predicted that a Bush veto of the family leave bill might provoke such a public outcry that supporters could override a veto, even though the House vote for the measure fell 46 votes shy of the two-thirds necessary. The Senate passed the measure on a voice vote.

"If the House is able to override, then I think we'd have a very good crack at it" in the Senate, Sen. Christopher J. Dodd (D-Conn.) said.

"This is a very precarious position for our party to be in," said Rep. Marge Roukema (R-N.J.), a key supporter of the measure. "It's clear cut that the Republican Party should be supporting this and that the first father, George Bush, should be supporting this. We've protected the legitimate concerns of small business. . . . It is a very popular issue with people, every poll shows that."