Defense lawyers also say the arguments ignore the proven power of DNA to refute almost every other type of evidence.

In a case before the Pennsylvania Supreme Court, for example, Lynne Abraham, the Philadelphia district attorney, argued that the defendant, Anthony Wright, was not entitled to DNA testing because of the overwhelming evidence presented at trial, including his confession, four witnesses and clothing stained with the victims’ blood that the police said was found at Mr. Wright’s home. The Pennsylvania DNA statute requires the courts to determine if there is a “reasonable possibility” that the test would prove innocence.

Prosecutors say they are concerned that convicts will seek DNA testing as a delay tactic or a fishing expedition, and that allowing DNA tests undermines hard-won jury verdicts and opens the floodgates to overwhelming requests.

“It’s definitely a matter of drawing the line somewhere,” said Peter Carr, the assistant district attorney who handled the case of Mr. Wright, who was accused of raping and killing a 77-year-old woman. The defendant did not request testing until 2005, three years after the statute was passed, Mr. Carr said, and in his view there was no possibility that the test would show innocence.

“There’s also the idea that you want finality for the victim’s sake,” Mr. Carr said. “If someone else’s semen was found at the crime scene, we’d have to talk to the victim’s family about whether the victim was sexually active.”

Barry Scheck, a co-founder of the Innocence Project, a New York legal advocacy group that uses DNA to help the wrongfully convicted, said that most prosecutors no longer resisted testing in cases like Mr. Wright’s, where there is one perpetrator. More obstacles arise, Mr. Scheck said, in cases with multiple defendants or cases where a test result might point to another suspect, even if it does not clearly prove the innocence of the defendant.

In one such case near Austin, Tex., a defendant who was convicted in the bludgeoning death of his wife requested a DNA test on a bloody bandanna found 100 feet from the house. On its own, a test of the bandanna would not prove the guilt or innocence of the defendant the same way testing semen in a rape case might. But if it matched DNA found at the scene of a similar crime in the same county, or DNA in a database of convicted felons, it would be significant evidence that someone else might be responsible  the kind of evidence that might plant a reasonable doubt in a juror’s mind or lead to a confession by a perpetrator.