Moore, who gained national attention for his legal fight to display the Ten Commandments in his courtroom, announced his plans to run Monday.
Riley began his campaign at a 61st birthday party. He did not mention Moore by name, but did address the issue of God and government.
"Some say they can no longer acknowledge God in government. I think that's sad. Because I acknowledge him every day — in speeches, in the office, in meetings, schools and churches. We can all do that every day in the way we live our lives," Riley said.
Riley's announcement "presents a clear choice for the future," Moore said. "We must return Alabama to the people and take away power from the special interests that continue to control the state."
The two will meet in the Republican primary on June 6, but the field may grow. State Sen. Harri Anne Smith (search) has said she may seek the nomination. On the Democratic side, Lt. Gov. Lucy Baxley (search) and former Gov. Don Siegelman are (search) seeking the nomination.
Riley, a former farmer and businessman, served six years in the U.S. House before defeating Siegelman by 3,120 votes out of 1.3 million cast in the 2000 election.
Merle Black, a political scientist at Emory University who specializes in Southern politics, said Riley's announcement sets up a classic Republican primary battle, with Riley appealing to "Chamber of Commerce Republicans" and Moore drawing support from the party's religious wing.
"The split in the Alabama Republican Party has been so pronounced for so long. It looks like one more round in an unending battle," Black said.