It is not uncommon for an incumbent politician to limit his exposure during debates when facing a primary challenger. A primary is, after all, the political semifinals. No one remembers who lost the party nomination.

But when the incumbent gained his current seat as a member of the other party, declining to debate candidates from the adopted party can become a liability.

That is the dilemma facing Sen. Arlen Specter, who won five terms in the U.S. Senate as a Republican before defecting to the Democratic Party last year.

Specter has agreed to a single debate with U.S. Congressman Joe Sestak, a retired U.S. Navy admiral with a doctoral degree in public policy from Harvard.

Sestak, who announced plans to run for the seat before Specter converted, has called for six debates - one in each media market.

One of the debates Sestak agreed to attend would be held at Franklin & Marshall College. Specter, however, has agreed to just a single debate on May 1 in Philadelphia to be broadcast statewide. Monday, he rejected a proposal to debate Sestak in Erie.

He should reconsider.

Specter says people can judge him by his 30 years in office. But those 30 years were spent as a Republican. During that time, he voted with Republican presidents 85 percent of the time. He owes members of his new party the opportunity to see if his political conversion represents a change in philosophy or if it was merely a political calculation that he could not beat former U.S. Congressman Pat Toomey in a Republican primary.

The Democratic Party's big guns have come out for Specter. President Barack Obama has appeared at fundraising events with the senator. Gov. Ed Rendell has endorsed him. That's not surprising. As a veteran of the Senate, he has a significant profile that could benefit Pennsylvania. He also has a history of working with Democrats.

Sestak says he understands why party leaders have endorsed Specter. But he says the voters need to look at who is the best Democrat.

That's a message party faithful understand. And in that regard, he may have an edge.

The party faithful are the voters who typically turn out for mid-term elections.

Another factor in Sestak's favor is Pennsylvania's closed primary. In the past, Specter has received significant support from moderate Republicans who, unless they register as Democrats, cannot vote in the primary.

Specter owes members of his new party the opportunity to hear how he differs from Sestak. He has an obligation to discuss his past support of Republican initiatives.

Unfortunately, Democratic Party leaders apparently have no desire to require Specter to explain himself. That's shortsighted.

It may serve Specter's interests, but does it serve the party's?

A single debate is better than none at all. But if Specter slides through the primary only to lose the general election because rank-and-file Democrats are not certain Specter represents their interests, party leaders will have only themselves to blame.