Mitt Romney - who ran for the White House in 2012 - is warning that Donald Trump is hiding some bombshell in his federal tax returns.
It's one of the many questions Republican leadership are posing, as they worry about the man who appears to be on track to be the party nominee.
The five remaining candidates debate later (2100 Eastern / 0200 GMT).
Our U.S. correspondent Priscilla Huff reports:
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