ALBANY – Westchester County’s tough and telegenic district attorney, Jeanine Pirro, said yesterday she will seek the Republican nomination to run against Senator Clinton next year. Mrs. Pirro’s decision ends months of speculation over which statewide office she would seek and triggers what could be the most visible and costly statewide race in the nation’s memory.

Mrs. Pirro, whose busy travel schedule over the past year fueled speculation she was eyeing higher office, announced in late May that she would not seek a fourth term as district attorney but would instead run for state attorney general, governor, or U.S. senator. As recently as last week, speculation grew that the veteran prosecutor had decided on the governor’s race, after Governor Pataki said he would not seek a fourth term next year.

Yet Mrs. Pirro, 54, made every effort yesterday to portray herself as unwavering, distinguishing herself from Mrs. Clinton in interviews and written statements as someone pledged to serve out her term “because the people of New York State deserve a senator who will … not miss votes to campaign in primaries.” Mrs. Clinton, 57, is expected to seek, and quite likely to win, the Democratic nomination for president in 2008, and she has expressed reluctance to make such a pledge.

Mrs. Pirro’s entrance into the race is a jolt to the political scene going into next year’s elections. By choosing against the race for attorney general, she leaves Republicans scrambling to find a candidate to run in that race against a crowded Democratic field. By choosing not to run for governor, she leaves a similar hole. New York’s current attorney general, Eliot Spitzer, has led all potential opponents in the governor’s race, including Mrs. Pirro, by double-digit margins for months.

Despite those Republican question marks, Mrs. Pirro’s decision to run against Mrs. Clinton is thought to be very good news for her party’s money raising efforts, and something of a headache for Mrs. Clinton. A striking woman with a sensibility toughened by years of prosecuting sexual predators and mobsters, Mrs. Pirro will be able to employ an aggressive style without opening herself up to the charge of bullying Mrs. Clinton – as the former first lady’s last opponent, Rick Lazio, then a Long Island congressman, was. Independent strategists said Mrs. Clinton’s advisers will probably try to keep the two women apart as long as possible.

Still, Mrs. Pirro faces an uphill battle, even before a possible head-to-head race against Mrs. Clinton. No Republican candidate in New York has won statewide office without Conservative Party endorsement since the mid-1970s, and Mrs. Pirro’s liberal stance on social issues will make it difficult for her to win that party’s backing in a field that includes other, more conservative candidates. A Manhattan attorney and son-in-law of Richard Nixon who also plans to run against Mrs. Clinton, Edward Cox, predicted Mrs. Pirro will not earn Republican or Conservative endorsement.

“Jeanine just isn’t going to be able to get the Conservative Party nomination,” a spokesman for the Cox campaign, Thomas Basile, said. “We’re building this campaign to win, and we know we can do it.”

The chairman of the Conservative Party, Michael Long, said Mrs. Pirro left a message on his answering machine yesterday morning stating her intentions. He said he has told her in the past that “it would be a tough lift” for her to win his party’s endorsement in a race populated with candidates who are more conservative than she. Mr. Long said he has not personally closed the door on any candidate, however, and acknowledged that Mrs. Pirro would be a strong candidate.

“There’s no question about it,” Mr. Long said. “She’s good on her feet. She’s a formidable candidate. But I think so is John Spencer and so is Ed Cox.” Mr. Spencer, a former mayor of Yonkers, is likewise considering a run against Mrs. Clinton.

Another potential obstacle for Mrs. Pirro is her husband, Albert Pirro, who was convicted of felony tax fraud, involving $650,000, in 2000. In addition, Pirro has fathered a child outside his marriage, and in 1986 his refusal to release details of his law practice prompted Mrs. Pirro to withdraw her candidacy for lieutenant governor of New York. Mrs. Clinton’s husband was impeached while president and has a well-publicized history of philandering, but he has never served time in a federal prison.

A primary fight among Messrs. Cox and Spencer and Mrs. Pirro would be a boost to Mrs. Clinton, with Republican candidates spending money and energy battling each other instead of her. In a sign that Republicans are already torn over the possibility of an internal fight, the chairman of the party’s state committee, Stephen Minarik, told The New York Sun he has commitments from all three potential candidates not to force a primary even as the Cox camp pledged that, if necessary, it would.

“We have always said that we are in the right place on the issues for New York and for the party, and that if we have to fight and win the primary, we will fight and win the primary,” Mr. Basile said.

A spokesman for Mrs. Clinton, Howard Wolfson, reacted stoically to Mrs. Pirro’s entrance into the race, declining to say whether he was displeased with her decision and saying he would not “presume to handicap” a race between the women. Mr. Wolfson did predict, however, that the campaign will be expensive and said he expects that whichever candidate the Republicans nominate will have the ability to raise a substantial amount of money.

“I think we have to presume it will be extremely expensive, and we will do what we can to be prepared,” Mr. Wolfson said. Asked whether he anticipated the race would be the most expensive ever, Mr. Wolfson said: “It’s not really a record anyone is looking to break.”

Mrs. Clinton’s first run for Senate was the most expensive such race ever, with the Democrat spending nearly $30 million and Mr. Lazio spending more than $40 million. Senator Corzine, a Democrat from New Jersey, spent a record $62.8 million on an individual Senate campaign five years ago when he ran against James Florio in the primary and Bob Franks in the general election.

Mrs. Pirro notified Republican officials, including Mr. Minarik, Sunday evening of her decision to run against Mrs. Clinton, and she notified individual news outlets yesterday morning. Her advisers issued a press release later in the day that indicated the type of campaign she intends to run: conservative on fiscal issues and crime, liberal on social issues such as abortion. Mrs. Pirro, who plans to kick off her campaign officially on a three-city, statewide tour tomorrow, is expected to portray Mrs. Clinton as a captive of the fringe elements within the Democratic Party.

“Jeanine Pirro is going to run as someone who is not beholden to the partisan infighting in Washington,” her chief political adviser, Kieran Mahoney, said. “She will be a uniter, not a divider. Hillary Clinton has had the opportunity to join those ranks and has chosen not to do so.”

In a sign that Mrs. Clinton’s roots outside the state will also be a campaign issue, Mr. Mahoney, a former top aide to Mr. Pataki, added: “We won’t be taking any bus tours to find out what New York looks like.”

Some political observers have expressed doubt that next year’s race against Mrs. Clinton will motivate Republicans nationwide to send money to the challenger, the way they did when Mr. Lazio raised tens of millions in the last four months of the 2000 race. Mr. Minarik, however, predicted a similar outpouring this time around.

“She’s still a wild-eyed liberal,” Mr. Minarik said. “People across the country are just sitting in their homes waiting to write a check to the Republican candidate for U.S. Senate from New York.”