The two rivals in the campaign for Manhattan district attorney have not yet formally declared their candidacies, but a dispute has broken out between their campaign staffs about whether the eight-term incumbent, Robert Morgenthau, will challenge the petitions circulated by his opponent, Leslie Crocker Snyder.

According to letters between representatives of the camps, members of Ms. Snyder’s staff say members of Mr. Morgenthau’s staff are violating an agreement they made in February not to challenge each other’s ballot signatures. In response, the Morgenthau camp says no promise was made and Mr. Morgenthau has yet to decide whether to mount a legal challenge to Ms. Snyder’s petitions.

Yesterday, Ms. Snyder’s campaign manager, Jackie Rovine, made public a letter she wrote to Mr. Morgenthau’s campaign manager, Eben Bronfman, in which she called Mr. Bronfman “cowardly” for abandoning an agreement the campaigns had reached not to challenge each other’s ballot signatures. A spokesman for Mr. Morgenthau, Robert Liff, said, “No promise was made, no deal was made,” and the “whole thing is made up of whole cloth.”

To get on the ballot for the Democratic primary for district attorney in September, a candidate must collect 4,000 signatures on nominating petitions. Because many signatures can be discounted by legal challenges, analysts suggest a candidate should collect at least three times the required minimum.

Mr. Morgenthau, who has never faced a strong electoral challenge while serving as chief prosecutor for three decades, has wide support within the party. In this year’s campaign, he has also won the endorsement of every Democratic club so far, and the clubs’ volunteers are considered invaluable in collecting signatures on candidates’ petitions. A former judge of state Supreme Court but a rookie in competitive elections, Ms. Snyder faces a hurdle in collecting thousands of signatures. A peace pact over signature challenges would benefit her.

Yesterday’s letter caps a series of exchanges over signature challenges. In a letter dated March 3, an attorney for Ms. Snyder’s campaign, Lawrence Mandelker, wrote to Mr. Bronfman to accept an offer, which Mr. Mandelker said was made to Ms. Rovine, to forgo signature challenges. On March 30, a secretary for Mr. Morgenthau’s campaign, John Fried, wrote back, saying the Snyder camp had been “misinformed” and no agreement was made.