ALBANY – Governor Pataki’s decision this weekend to veto a bill that would expand access to the “morning after” pill triggered a fierce reaction among abortion-rights advocates yesterday, with many vowing to make the governor’s expected presidential bid even more difficult than it was already thought to be.

The governor’s office said late Sunday Mr. Pataki would veto the controversial bill on emergency contraception drugs in the form in which it was passed, because it does not include protections for minors. Abortion-rights leaders, who said the governor never indicated a concern about age in the past, cast his veto threat as a political stunt aimed at making him palatable to national Republicans. They threatened in return to help sabotage that effort if the governor blocks the bill.

“I think he saw us as an isolated New York group, and that is not the case,” the executive director of NARAL Pro-Choice New York, Kelli Conlin, said. “We will be calling on the resources we have at hand so the people in Iowa and New Hampshire know what New Yorkers have learned: that Governor Pataki has abandoned the people of this state and can’t be trusted.”

Ms. Conlin’s group was planning an advertising blitz in those first states to select presidential nominees, and in New York City, urging Mr. Pataki to sign the contraception bill. In the NARAL advertisement, to be shown on television this week, a female narrator says: “As he considers the Oval Office, he may want to consider this. New Yorkers and Americans value principles over politics. Do the right thing, Governor.” Ms. Conlin told The New York Sun her group will likely increase spending on the ad campaign and tweak its message following Mr. Pataki’s decision.

The governor’s decision to veto the bill comes less than a week after he announced he would not seek another term in office next year and two weeks after he traveled to Iowa to test the presidential waters. Political analysts said Mr. Pataki’s greatest weakness going into a possible national campaign is his support as governor for liberal social causes such as gay rights, abortion rights, and gun control.

A longtime proponent of access to abortion, Mr. Pataki had not taken a position on a minor’s access to emergency contraception until now. Reactions were mixed on what a veto may mean for him politically. The governor of Massachusetts, Mitt Romney, another Republican viewed as a presidential aspirant, vetoed a similar bill last week and now faces an override. An override is unlikely in New York, where the Republican-majority Senate passed the bill on emergency contraception only narrowly.

One veteran political strategist, Hank Sheinkopf, said the veto would probably help Mr. Pataki politically outside New York. “I think politically, he’s got to do something that moves him more to the center, so from that perspective he did the right thing,” Mr. Sheinkopf said. “Whether it was the right thing to do morally is another question.”

Another strategist, Rich Galen, was more skeptical about the veto’s potential political efficacy.

“You can’t finesse this,” Mr. Galen said. “Anybody who tries to finesse this issue immediately becomes suspect.”

A vice president for governmental affairs at the conservative Family Research Council, Connie Mackey, said a candidate’s actual position on issues matters more than his previous statements or long-term record.

“I would say that when politicians get to Washington, they are known to grow in office, and that usually means to the left,” Mrs. Mackey, whose group is based in Washington, said. “If Governor Romney and Governor Pataki are seeing that the tide is turning and are growing in our direction, then they are growing in the right direction. I would say he’s learning and growing in the right direction.”

The veto decision was a clear political gamble: Mr. Pataki has never been known to rankle abortion-rights groups during 11 years as governor, and his new position on the contraception issue made him an instant enemy of that constituency. The governor has long pushed for expanded access and increased public financing for abortions and contraceptives in New York, but abortion-rights groups all but wrote him off as an ally yesterday.

“What people hate in a politician,” Ms. Conlin said, “is someone who just spins in the wind. And by abandoning his record in New York, the governor clearly does that.”

In calling Mr. Pataki’s decision a reversal, abortion-rights groups point to legislation he has signed on a variety of issues. Ten years ago, he signed legislation that provided Medicaid funds for abortions for women of any age. Four years later, he made it a crime to interfere with access to abortion clinics. In 2001, Mr. Pataki became the first governor in America to approve Medicaid coverage for the abortion drug RU-486. And two years ago, the governor signed a law that required emergency rooms to provide emergency contraception drugs for all victims of rape.

“He’s got a 10-year record in New York State on women’s health which is a good record,” the head of Family Planning Advocates of New York, JoAnn Smith, said. “So this is a 180-degree turnaround. I think for any elected official to turn around on what appeared to us to be a core belief makes it very hard for them to explain what they really do believe.”

Despite the broadside from abortion-rights groups, aides to the governor said the issue of parental notification has not previously come up in abortion-related legislation during Mr. Pataki’s tenure. They pointed to a gubernatorial debate from 1998 in which the governor stated, in answer to whether he supports parental notification for abortions, “Under current law, those seeking that service are able to find it. I support parental notification.”

Nor was support for Mr. Pataki’s move unanimous among opponents of abortion rights, however.

An opponent of abortion rights, William Donohue, president of the Catholic League for Religion and Civil Rights, doubted that Mr. Pataki’s stance was divorced from presidential politicking.

“I hate to agree with NARAL on anything, but obviously there’s a lot of truth in what they are saying,” Mr. Donohue said. He said opponents of legalized abortion commend Mr. Pataki’s positions in favor of parental notification and against the morning-after pill and partial-birth abortions, but suspect his principles have not shifted for good.

“Is this the real George Pataki, and if so, is he going to change is position on abortion in general? Or is he just going to flip-flop on the issue?” Mr. Donohue said. “So certainly, this does smack of political opportunism.”

Pataki aides, meanwhile, left open the possibility yesterday that he would sign the bill if lawmakers agree to make changes to it before sending it to his office Thursday.

“Consistent with his record on women’s reproductive issues, the governor plans to veto the legislation primarily because it provides no protection at all for minors,” a Pataki spokesman, Kevin Quinn, said. “If this and other flaws in the bill are addressed, and a responsible version of the bill is advanced, the governor would support it.”