On Sunday, Carson took a small step toward joining Senators Ted Cruz, Rand Paul and Marco Rubio as a 2016 presidential candidate.

The Republican field alone could feature a dozen candidates, though questions remain about whether New Jersey Governor Chris Christie will challenge Carson, the quasi libertarian Paul, Floridian Rubio and Cruz, born in Canada to Cuban parents but now a legislator for the US state of Texas.

"I'm willing to be part of the equation and, therefore, I'm announcing my candidacy for president of the United States of America," Carson said Sunday in an interview with the Florida CBS affiliate WPEC-TV.

Media have predicted that the eventual Republican nominee will face former Secretary of State Hillary Rodham Clinton, a Democrat, in the vote. Some say such fatalism is unfair to Clinton's own first official primary opponent, Bernie Sanders, who arrived in Congress in 1991 pledging to fight growing economic disparity, improve health care and education, and reel the United States in from its involvement in "unnecessary" foreign wars.

Potential candidates

The candidacy of another potential Republican contender was called into question over the weekend. On Friday, federal prosecutors charged three of Republican New Jersey Governor Christie's allies in a scandal over traffic created in the small city of Fort Lee, where the Democratic mayor had failed to endorse the incumbent Christie in his ultimately successful 2013 re-election bid. The governor claimed vindication.

"I had no knowledge or involvement in the planning or execution of this act," Christie said.

Another buzzed-about but undeclared Republican is former Florida Governor Jeb Bush, who, if elected, could become the third in his family to rule the US in a quarter century. There's also current Wisconsin Governor Scott Walker, whose union-busting tactics have made him a darling of the American right.

Whoever emerges with the Republican nomination would likely face Clinton, a former secretary of state, senator and first lady who has tried to shift to a populist stance in recent weeks. Running to Clinton's progressive left, Vermont Senator Sanders wants to end tax breaks for the very wealthy.

Sanders also fiercely opposes the Trans-Pacific Partnership - a huge trade pact under negotiation with countries in and around Asia - saying the deal would cost "millions of decent-paying" American jobs. Several other Democrats, including former Maryland Governor Martin O'Malley and former Virginia Senator Jim Webb, have expressed an interest in running.

mkg/cmk (Reuters, AFP, dpa, AP)