Has Hillary Clinton finally met her match?

You don't need any trailers to tell you that this will be the blockbuster of 2006. It will have a monster budget - some expect this to be the most expensive Senate race in history - and a gripping subplot too. (A quick clue here: Hillary-for-President 2008.) And then there is Ms Pirro, charismatic,outspoken and popular.

The formal announcement from Ms Pirro, 54, just completing her third term as the District Attorney for Westchester County, just north of New York City, is expected today. However, she let the cat out of the bag to reporters earlier this week. She wants to be the Republican to take on Hillary next year and she means to win.

That may be a little ambitious, of course. Near the end of her first term, Ms Clinton has attained considerable popularity, even in the northern parts of the state, which are normally considered more conservative. A recent poll suggested that in a race with Ms Pirro, she would prevail by 63 per cent to 29 per cent.

On the other hand, the vote is still one year away. Republican strategists in Washington have been prodding Ms Pirro to take the Senator on. They do not completely rule out that she could win and score the political upset of the decade. Failing that, they calculate that she, more than anyone else available, may be able to wound Mrs Clinton sufficiently to slow her momentum going into 2008 and the next race for the White House.

For her part, Mrs Clinton, 57, is saying little. She especially refuses to speak about her presidential ambitions, insisting she is focused only on the Senate race. Yet, it is widely accepted that she is better positioned than any other Democrat at the moment to win the nomination to run for the White House next time around.

At least two other Republicans have expressed interested in challenging Mrs Clinton, which could force Ms Pirro into competing in a primary race first. One of the other would-be candidates is Edward Cox, a Manhattan lawyer and the son-in-law of the late former president Richard Nixon. But now that her hat is in the ring, Ms Pirro is almost certain to get her party's nod.

Already, Ms Pirro is probing what may be the biggest weakness in Mrs Clinton's re-election platform - her inability or unwillingness to close her presidential options by pledging to serve all six years of a second Senate term if voters send her back to Washington.

"When Mrs Clinton first came to us and said she wanted to be a New Yorker, she asked New York to put out a welcome mat and we did," Ms Pirro said in a statement. "But now she wants us to re-elect her even though she won't promise to serve out her term and wants to use us as a springboard to the presidency. She's asking us to become her doormat."

She amplified the theme talking later with reporters. "There is only one woman who really wants the job and who is looking to serve out the term - and that's me," Ms Pirro said. "Hillary Clinton is not running to serve the people of New York. We are a way station in her run for the presidency."

Ms Pirro may be relatively unknown on the national stage, but in New York State she has long been a bright, if sometimes controversial, star in her party. As chief prosecutor in Westchester County (home to Hillary and Bill) she has made a name for herself combating domestic violence and pursuing sexual offenders. She has also carved out a niche as a legal pundit on Fox News.

However, she faces some obstacles, such as the legal difficulties of her husband who recently served an 11-month prison sentence for tax fraud. Perhaps more importantly, she is a moderate Republican who supports the death penalty but is also pro-choice on abortion and supportive of gay rights. This could cost her the backing of the more traditional conservatives.