Mitt Romney questions Donald Trump tax situation
Republican presidential frontrunner Donald Trump has a "bombshell" hidden in his tax returns, the party's 2012 nominee Mitt Romney has warned.
Mr Romney says the billionaire was "dodging and delaying" on releasing his returns, which could shake up the race.
Mr Trump has hit back calling the ex-governor "one of the dumbest and worst candidates" in Republican history.
Donald Trump swept to victory in Nevada on Tuesday, his third straight win in the Republican race for nomination.
The 69-year-old easily defeated his rivals, Marco Rubio, Ted Cruz, John Kasich and Ben Carson, ahead of the all-important "Super Tuesday" next week when 11 states are due to decide on their party candidates and a quarter of all nominating delegates are up for grabs.
All five Republican candidates are set to square off in another presidential debate in Houston, Texas, later on Thursday.
Analysis - Anthony Zurcher, BBC News, Houston, Texas
Donald Trump now has a hat-trick of insults lobbed against former Republican presidential standard-bearers.
First it was his spat with 2008 nominee John McCain, in which he questioned his heroism as a prisoner of war during Vietnam. Then it was George W Bush and his actions before and after the 9/11 attacks. Now Mr Trump is mixing it with 2012 Republican candidate Mitt Romney - and like the other two battles, the New York billionaire will probably end up on top.
While Mr Romney is still largely well liked in the party, Mr Trump's view that he made tactical errors that let Barack Obama win an ill-deserved re-election is widely shared among disaffected voters now backing the New Yorker. This makes any attacks from Mr Romney less effective.
By questioning Mr Trump's tax returns, Mr Romney seeks to call into question a key part of Mr Trump's political CV - his business success.
The success of this move seems unlikely, however, given that while tax forms may contain some unflattering information, Trump fans need only look at the skyline of New York, Las Vegas and elsewhere - with their glittering Trump-emblazoned skyscrapers - for more tangible evidence of the billionaire's success.
"I think we have good reason to believe that there's a bombshell in Donald Trump's taxes. I think there is something there," Mitt Romney told Fox news on Wednesday.
This is because, he said, every time Mr Trump is asked about his taxes "he dodges and delays".
"We could find he doesn't have anywhere near as much income as we might think he would have with a $10 billion net worth, or he doesn't pay any taxes or he pays very, very low taxes," Mr Romney added.
The former governor of Massachusetts, who has yet to endorse a Republican presidential candidate, came under scrutiny from Democrats about his own financial dealings and taxes during his failed 2012 campaign.
Under intense pressure, he ended up releasing two years' worth of tax returns at the start of the primary race that year.
Donald Trump, who endorsed Mr Romney in 2012, was quick to respond to the comments in a series of tweets: "Mitt Romney, who totally blew an election that should have been won and whose tax returns made him look like a fool, is now playing tough guy."
Mr Trump says he will release his tax returns - which he says "are extremely complex" - eventually, amid calls to do so from his rival, Texas Senator Ted Cruz.
Forbes estimates the real estate mogul's net worth to amount to $4.5bn (£3.2bn), though he himself puts the figure much higher at $10bn.
Mr Cruz, Ohio Governor John Kasich and Florida Senator Marco Rubio have all vowed to release their tax returns soon.