The prospect of an independent U.S. Senate candidacy by Kate Michelman, a veteran abortion rights activist, poses a threat to Democrat Bob Casey Jr. but is one that would face formidable obstacles before it could become reality.

Ms. Michelman has been critical of national and state Democratic leaders for uniting behind Mr. Casey, the state treasurer, as a challenger to Sen. Rick Santorum, R-Pa.

The former president of NARAL Pro-Choice America, the abortion rights group, took particular exception to Mr. Casey's statement earlier this year that he would have voted to confirm Justice Samuel Alito's nomination to the Supreme Court.

"For me and for many people across the country who care about women's rights, that was the straw that broke the camel's back,'' Ms. Michelman said. "I have become a vehicle for people who feel they have to take action at a time of crisis.''

Ms. Michelman, who has been making appearances around the country to promote her memoir, "With Liberty and Justice for All,'' said she was seriously considering the race and planned to decide in the near future.

"I am talking to a number of close friends and political advisers,'' she said. "I was appalled at the lack of commitment by pro-choice senators [in the Alito confirmation]. I was particularly appalled by Bob Casey's statement, which I thought was politically unwise and unnecessary.''

The Casey campaign greeted the Michelman speculation with a conciliatory observation: "Democrats are stronger when we work together on the many issues where we have common ground and Bob Casey looks forward to doing that in the U.S. Senate."

Ms. Michelman's decision comes as the always volatile abortion issue has been amplified by two high court confirmation battles and moves by legislatures in South Dakota and Mississippi to enact anti-abortion measures calculated to force the Supreme Court to revisit the landmark Roe v. Wade decision.

Mr. Casey has held a consistent lead over Mr. Santorum in early polling in the Senate race. But those same surveys have suggested that both are more conservative than the majority of Pennsylvania voters, particularly the majority of Democrats on the abortion issue.

The other side of that ideological coin, and one of the chief rationales for Mr. Casey's candidacy, is that his pro-life stance neutralizes an issue on which Mr. Santorum and other Republicans have been able to make political inroads among socially conservative voters, including many Democrats.

A Quinnipiac University survey in December reported that 22 percent of Mr. Casey's initial Democratic supporters changed their minds and said they would not vote for him when informed of his pro-life views.

The same poll reported that 62 percent of the Democratic voters contacted said that abortion should be legal in all or most cases. Fifty-three percent of all voters were in the same category, while 42 percent of the total sample and 33 percent of the Democrats answered that abortion should be illegal under most or all circumstances.

For Ms. Michelman or any independent to exploit that issue by taking an independent spot on the November ballot would require a huge logistical leap. Pennsylvania has some of the most onerous rules in the nation for independent or third-party candidates.

To gain a spot on the ballot, state law requires that an independent candidate submit nominating petitions with total signatures from registered voters equivalent to 2 percent of the number of votes cast for the highest vote-getter in the previous statewide election. That would mean that Ms. Michelman or any other prospective independent would need just over 67,000 signatures -- 2 percent of the vote cast for none other than Bob Casey in the 2004 election for state treasurer.

While he couldn't have planned it this way, Mr. Casey's landslide victory is paying added dividends now by raising the signature bar for would-be independent rivals in November. He received more than 3.3 million votes in the 2004 contest, the highest total in Pennsylvania history, and a result that greatly enhanced his attractiveness to national Democrats seeking a candidate to oust Mr. Santorum.

By comparison, Ralph Nader, despite the advantages of a well-known name and paid staff, failed to gather the roughly 26,000 signatures he would have needed to get on the state's 2004 presidential ballot -- the figure being lower because of the lower turnout in the 2002 election, when the governor's race was at the top of the ballot.

Both Mr. Santorum and Mr. Casey face pro-choice candidates in the May primary.

Reacting to the reports of Ms. Michelman's possible run, Chuck Pennacchio, one of Mr. Casey's opponents for the Democratic nomination, warned that a third-party, pro-choice candidate would divide Democrats, weakening the chances of ousting the incumbent.

Alan Sandals, who also favors abortion rights, also is seeking the party's nomination.

Ms. Michelman said that she was not considering a run in the Democratic primary because the Tuesday deadline for nominating petitions to get on that ballot was so soon.

"The primary is right around the corner,'' she said, while acknowledging the stiff challenge of an independent bid. "I know that if I'm going to do this, I've got to get moving. I understand that.''