"If he's giving checks to Democratic politicians and he supports their views... then it starts to suggest, 'gosh if he publicly supports their views if he finances their causes,'" Cruz told Boston radio station WRKO. "Then suddenly when he announces as a candidate for president, every single one of his views changes — listen if he has had a change of heart, I am thrilled."

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Cruz, who is competing with Trump for the GOP nomination, added that if Republican voters want a nominee who will "cozy up to Chuck Schumer — by the way he's written checks to Chuck Schumer, I never have — then you ought to be backing him."

The Texas Republican's comments come after Trump told ABC's "This Week" that Cruz is a "nasty guy" and "nobody likes him. Nobody in Congress likes him."

The two candidates have been locked in an escalating rhetorical battle this month after Cruz said that Trump represents "New York values."

Trump, in return, hit Cruz over previously undisclosed loans he accepted from Goldman Sachs and Citibank to help finance his Senate run. Cruz, on Monday, suggested that Trump's criticism was hypocritical, calling it "more than a little rich."

"You want to assess who stands with Wall Street? Then look at the actual records of the candidates," he added. "Donald Trump vocally and enthusiastically supported TARP, the bailout of the big banks. He vocally and enthusiastically supported Barack Obama's stimulus plan."

Trump suggested that the loans underscored that the senator is controlled by the banks.

While Cruz had long been reluctant to take on Trump directly, the two have stepped up their rhetorical barbs as they head toward the Feb. 1 Iowa caucus.

Trump has led the Republican field in recent Iowa polling, but Cruz claimed the top spot in a Des Moines Register/Bloomberg poll released last week.

Trump is planning to campaign in Iowa this week with stops scheduled for Tuesday, Wednesday, Saturday and Sunday.

While Cruz on Monday urged primary voters to study candidates' records, he sidestepped directly calling Trump a "fraud" or a "hypocrite."

"Those are your words," he told radio host Jeff Kuhner, adding, that he doesn't plan to get into a "food fight or personal insults."

However, he noted that "Donald seems very rattled right now. ...He's been launching an awful lot of attacks in the last couple of days."