(CNN) Donald Trump is striking back at Mitt Romney on Thursday, after the 2012 Republican presidential nominee said his party's current front-runner could have a "bombshell" in his tax returns. "Mitt Romney, who was one of the dumbest and worst candidates in the history of Republican politics, is now pushing me on tax returns. Dope!" the billionaire businessman said in a series of Tweets attacking Romney. 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.Donald Trump said he will decide whether to release his tax returns and called Mitt Romney "yesterday's news"The 2012 Republican nominee's broadside followed Trump's thumping victories in three of the first four GOP nominating contestsRomney warned Wednesday that Donald Trump's tax returns could contain a "bombshell"
Opinion: It's over; _ is going to be the Republican nominee.