JACKSONVILLE, Fla. — The Rev. Eugene W. Diamond of the Abyssinia Missionary Baptist Church here rarely pays attention to the clock when the spirit moves him to preach, but midsermon on Sunday, he said something unusual to his flock of hundreds: “Timing is critical, so let me hustle.”

He had already scaled back the minutes devoted to worship. Congregants had been instructed to forget wearing their Sunday best in favor of comfortable shoes because they all had work to do: moving thousands of “souls to the polls.” And they had only one Sunday to do it.

Mr. Diamond stressed the urgency in a tambourine-shaking, trumpet-blaring finale to his prayers: “Bless us as we make our voices heard!” he shouted above the music. “Let’s go! Let’s go! Let’s go! Let’s go!”

Across Florida, black churches have responded with ferocity to changes that Gov. Rick Scott, a Republican, and the Legislature made to eliminate six days of early voting this year — including the Sunday before Election Day, which had been the traditional day to mobilize black congregations. In 2008, black voters cast early ballots at twice the rate of white voters, and turned out in significant strength on the Sunday before Election Day to help propel Mr. Obama to victory here.