Philadelphia ? For decades, Pennsylvania has perfected a unique politics of ambivalence in its Senate races, sending to Washington nonconformists who do not fit comfortably into their parties’ norms but reflect accurately the subtle variations within their constituencies.

For 30 years, Arlen Specter has been the emblematic figure — a man who started as a Democrat, became a Republican for most of his political life, and then switched back. He was notorious for his “flexibility” on policy. A labor leader here once told me he had mockingly congratulated Specter for “staying on the same side of the debate all the way from breakfast to dinner.”

But as a local campaign consultant remarked this week, “Specter was not so different from the others” voters chose to represent them. With rare exceptions — such as Milton Shapp, a liberal Democrat, and Rick Santorum, a conservative Republican, who rose quickly and faded just as fast — successful politicians here have hugged the center line at the expense of ideological clarity.

But Specter’s fate signals that the era of ambivalence may be ending. Six years ago, he barely survived a Republican primary against Rep. Pat Toomey, a card-carrying conservative. Facing Toomey again in what would likely have been a losing race, Specter switched his allegiance back to the Democrats, only to run head-on into Rep. Joe Sestak, a suburban liberal who had been recruited for the House four years ago by Rahm Emanuel.

Now, Toomey and Sestak are squaring off for a showdown that presents the clearest of choices but leaves thousands of independent-minded voters wondering where to go.

On paper, Sestak, 58, and Toomey, a decade younger, are perfectly primed for what ought to be a great debate. Both well-educated and articulate, they have built their careers as idea merchants. Sestak is a graduate of the Naval Academy and served as commander of the George Washington aircraft carrier battle group and later as director of defense policy on President Clinton’s National Security Council. He retired as a rear admiral and, in 2006, ousted a Republican incumbent to become the highest-ranking veteran in Congress.

Toomey is a Harvard graduate who became a currency trader on Wall Street, won an open Democratic district in the Lehigh Valley in 1998 and served three terms in the House before stepping down in 2004 to fulfill his term-limits pledge. That year, he challenged Specter from the right and lost by less than 2 percent of the votes in the Republican primary. He then became the head of the Club for Growth, a free-market advocacy group that backs conservative candidates.

Sestak and Toomey are on opposite sides on most big issues. Sestak campaigned against the war in Iraq, supported Hillary Clinton for the 2008 Democratic presidential nomination, voted with President Obama on the stimulus bill, health care and the cap-and-trade energy bill, and has a 100 percent rating from the abortion-rights group NARAL Pro-Choice America.

Toomey argues for extending all the Bush tax cuts, opposes the three main Obama initiatives, is pro-life and against gay marriage and wants to eliminate the estate tax and reduce several business levies. He has a lifetime approval rating of 97 percent from the American Conservative Union.

But neither man appears comfortable betting his political future entirely on ideology. While happy to point to examples of the opponent’s “rigid ideology,” they also like to fuzz their own records. Sestak told me that, as a military man, he regarded “accountability” as the main issue. Seeking to make the campaign a test of character, he brought in New York City’s independent mayor Michael Bloomberg and former Republican Sen. Chuck Hagel of Nebraska, a fellow veteran, for endorsements.

He also tried to get TV stations to remove an ad claiming he was “100 percent” in agreement with Speaker Nancy Pelosi, citing studies showing that they actually have voted the same 97 percent of the time.

Toomey brought in moderate Republican Sen. Susan Collins of Maine for a fundraiser and told an interviewer he would have voted to confirm Justice Sonia Sotomayor for the Supreme Court. He also held out the prospect that if Republicans recapture Congress, Obama would react as Bill Clinton did in 1994 and “start to govern from the center. I would welcome that,” he said.

Viewers can expect two months of ads arguing that the other guy is the extremist.

— David Broder is a columnist for Washington Post Writers Group. davidbroder@washpost.com