A congressman whose constituents include the family of the longest-held American prisoner in Iran said Thursday that he had decided to vote in favor of the nuclear agreement reached with Iran on July 14.

The congressman, Dan Kildee, a Michigan Democrat who has become an outspoken advocate for the prisoner, Amir Hekmati, and other Americans held or missing in Iran, said in a telephone interview that he had informed President Obama of his decision on Wednesday night. He also said he had informed the Hekmati family, who live in Flint.

“For me, once I know where I am, there’s no sense in prolonging the question,” Mr. Kildee said. “I do think it’s important that we evaluate the agreement on its own merits, and the case of the Americans is a consideration but shouldn’t be determinative.”

Mr. Kildee’s assessment of the nuclear accord is important because the debate in Congress over whether to support it has become entangled with the prisoner issue. How he decides to vote could help sway others.