President Barack Obama's endorsement of gay marriage could chip away at his overwhelming support among African-Americans, Bishop Harry Jackson has said.

Jackson, a minister at the Hope Christian Church in Beltsville, Maryland, came to prominence fighting against the District of Columbia's gay marriage law and a similar law approved earlier this year in Maryland.

Appearing Friday on Fox News' The O'Reilly Factor, Jackson told guest host Juan Williams that George W. Bush won reelection in 2004, when marriage was an issue, because of a slight uptick in black support for Republicans in Ohio and Florida. He said presumed GOP presidential candidate Mitt Romney could reach blacks with the issue.

“Twenty percent of blacks will not vote for Obama” because of his stance on marriage, Jackson said.

The Rev. Conrad Tillard, senior minister of Nazarene Congregational Church in Brooklyn, New York, disagreed, saying black voters have never been single-issue voters.

“It comes down to a choice, we know what the fat cats from Bain and Wall Street have as an agenda for America. At the end of the day, we have a choice between the Democrats, who tend to consider all votes, particularly those stuck at the bottom,” Tillard said.

(Watch the entire segment at