Republican United States presidential frontrunner Donald Trump has a "bombshell" hidden in his tax returns, the party's 2012 nominee Mitt Romney has warned.
Mr Romney said the billionaire was "dodging and delaying" on releasing his returns, which could shake up the race.
Mr Trump - who swept to victory in Nevada on Tuesday, his third straight win in the Republican race for nomination - hit back, calling the former governor "one of the dumbest and worst candidates" in Republican history.
The 69-year-old overshadows rivals Marco Rubio, Ted Cruz, John Kasich and Ben Carson ahead of the all-important "Super Tuesday" next week when 11 states would decide on their party candidates with a quarter of all nominating delegates up for grabs.
All five Republican candidates were set to square off in another presidential debate in Houston, Texas, later on Thursday.
"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 $US4.5b, though he himself puts the figure much higher at $US10b.
Mr Cruz, Ohio governor John Kasich and Florida senator Marco Rubio have all vowed to release their tax returns soon.
The spat with Mr Romney rounds out a hat-trick of insults against former republican presidential flag-bearers, having questioned 2008 nominee John McCain's heroism as a prisoner of war during Vietnam and targeting George W. Bush over his actions before and after the September 11 terror attacks.