Trump wrote that tax returns have zero to do with a person's net worth, and all swung at the establishment wing of the Republican party, many of whom have coalesced behind Florida Sen. Marco Rubio as an answer to Trump and Texas Sen. Ted Cruz.
"Why doesn't @MittRomney just endorse @marcorubio already. Should have done it before NH or Nevada where he had a little sway. Too late now!"
Romney used the same medium to respond, tweeting back at the billionaire businessman Thursday.
"Methinks the Donald doth protest too much. Show voters your back taxes, @realDonaldTrump. #WhatIsHeHiding," Romney tweeted
The real estate mogul first rejected Romney's accusation out of hand in an interview with CNN's Anderson Cooper on Wednesday, saying "there is no bombshell at all other than I pay a lot of tax and the government wastes the money."
Romney's biting attack hinted at clear signs of alarm in the Republican establishment at the billionaire's tightening grip on the party's presidential race.
"We have good reason to believe that there's a bombshell in Donald Trump's taxes," Romney told Fox News, and also called on the top anti-Trump contenders Sens. Ted Cruz and Marco Rubio to disclose their tax information as well.
"Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying taxes we would expect him to pay or perhaps he hasn't been giving money to vets or to the disabled like he's been telling us he's been doing," Romney added.
Other Republicans -- including South Carolina Sen. Lindsey Graham
-- came to Trump's defense Thursday.
"We as Republicans should not accept that this man has a tainted financial past," Graham told CNN's "This Hour," adding later, "It's clear to me he doesn't want to show us his financial situation."
The 2012 Republican nominee's broadside followed Trump's thumping victories in three of the first four GOP nominating contests, including in Nevada on Tuesday night, which have established the billionaire businessman as the party's undisputed front-runner.
It was also an attack steeped in irony, since Romney was on the receiving end of similar claims by then-Senate Majority Leader Harry Reid
, when Democrats eviscerated him over his fortune and business record during the 2012 campaign. Reid zeroed in on his taxes, saying he hadn't paid any in a decade without offering any evidence to support the claim.
Trump left the door open to not releasing his tax returns Wednesday.
The GOP front-runner told CNN that he will "make a determination over the next couple of months" as to whether he will release his tax returns.
Romney, who decided against a third presidential run last year after initially considering jumping in to the race, uncorked his attack on Trump, ahead of next week's Super Tuesday contests that could further cement the billionaire's strong front-runner status in the GOP presidential race.
Romney's move appeared to not just be a sign of concern that Trump -- after defying pundits and political logic since he launched his "outsider" campaign last summer -- could actually go on to claim the Republican nomination.
It was also a sign of skittishness about the damage the Democrats will try to inflict on Trump, who has a long and sometimes controversial business record, in a general election, if he does indeed emerge as the GOP nominee.
"They were all over me for my taxes," Romney told Fox.
While Cruz and Rubio have yet to release their tax returns this campaign season, years of tax returns from both candidates are already publicly available from the time when they ran for Senate.
Reid ignited a firestorm in the 2012 presidential race by claiming on the Senate floor, without presenting any evidence, that "the word's out that he hasn't paid any taxes in 10 years." His attack was part of a fierce effort by the Obama re-election campaign to portray Romney as an out of touch and heartless businessman unable to understand the economic problems afflicting the middle class. Reid later told CNN that he did not regret his move, noting archly that Romney did not win the election.
The irony of the moment was not lost on Reid.
"All I know, I can't imagine Romney having the gall coming after anybody's returns," the Senate minority leader told CNN on Thursday. "Let's look at his."
It also struck one key member of Obama's re-election campaign.
"Did Mitt Romney just do to Donald Trump on tax returns what he was so mad at Harry Reid for doing to him?" said former Obama political adviser and current CNN commentator Dan Pfeiffer on Twitter.
Under intense pressure, Romney did finally release his tax returns during the campaign, but when it emerged that he had paid around 14% taxes on his 2010 return, there was political uproar that played into the hands of the Obama campaign.
The former Massachusetts governor was taking advantage of rules in the tax code under which income derived from dividends and capital gains is taxed at lower rates than traditional wages.
Trump last summer released his personal financial disclosure shortly after announcing his presidential run and has consistently touted the fact that he released his financial numbers ahead of schedule.
Trump said he was worth $8 billion, a figure he and his accountants later revised to $10 billion when he officially released his personal financial disclosure. Forbes has estimated Trump's net worth at $4.5 billion, a figure Trump has disputed.
On Wednesday, Trump also stressed that his tax returns "are extremely complex," which Romney has rejected given that Trump would only need to publish several years of past tax returns which he has already filed.
Trump stressed as he has in the past that he pays "as little as possible because it's an expense and it's not one I'm happy paying because frankly the United States government wastes a lot of money."