Everybody has heard of lemmings, the animals who sometimes hurl themselves off cliffs. That instinct for self-extinction also runs deep among people who call themselves progressive. We might call them the Demmings, because their grand gestures of principle hurt Democrats.

Kate Michelman, former president of the abortion rights group NARAL Pro-Choice America, has been sounding a lot like a Demming. Because Democrat Bob Casey came out in support of Supreme Court nominee Samuel Alito, she says she has been moved to consider running as an Independent against Republican Sen. Rick Santorum and likely Democratic challenger Bob Casey Jr., both of whom oppose abortion.

It would be hard for Ms. Michelman to get on the ballot. She would need signatures totaling just over 67,000 -- 2 percent of the votes cast for the highest vote-getter in the previous statewide election, which happened to be Mr. Casey in the 2004 election for state treasurer.

Ralph Nader, another Demming-type whose place on the ballot in 2000 was as responsible as anything for delivering the White House to George W. Bush, could not raise 26,000 Pennsylvania signatures in his second attempt at folly. Even if Ms. Michelman does not run, the very fact that she has been talking up a challenge is potentially harmful to Democratic chances.

Sen. Santorum's campaign had urged Mr. Casey to declare himself on the Alito nomination -- which had the makings of a clever trap. If he had said nothing or had opposed the nomination, his hope of recruiting some of Sen. Santorum's pro-life supporters would be jeopardized.

As it was, answering "yes" had its dangers, but for the trap to be properly sprung it needed someone to come along and loudly trip the wire. Ms. Michelman has obliged and Sen. Santorum must be rubbing his hands together with glee.

It's worth reminding readers that the Post-Gazette supports abortion rights and opposed the nomination of Samuel Alito. We would also note that a U.S. senator has influence and voting power over a wide range of subjects.

Yes, pro-choice advocates were disappointed when pro-choice Barbara Hafer withdrew as a candidate in this race, but abortion rights make up just one issue that affects the lives of women. Those who would scorn Bob Casey for his support of one nominee -- who has yet to rule on this issue as a justice -- take a remarkably shortsighted view. But that is the way of Demmings, who rush to the edge of the cliff and see nothing but blue sky.