(Reuters) -Former President Donald Trump, who has refused to debate any of his rivals for the Republican nomination, on Monday said he wanted to debate U.S. President Joe Biden immediately. Although the overwhelming frontrunner in the Republican race to challenge Biden in the Nov. 5 election, Trump has yet to sew up the nomination and has turned down Republican rival Nikki Haley's request that he debate her. In response to Trump's comments, Haley's campaign said in a statement that he was "too chicken" to debate her.