Republican White House hopeful Jeb Bush on Wednesday challenged rival Donald Trump's conservative credentials, saying the real-estate tycoon "was a Democrat longer ... than he was a Republican," NBC News reported.

The former Florida governor, himself often scrutinized by the GOP establishment for his relatively moderate views on immigration and social issues, offered what the network dubbed his "most pointed critique" of Trump as he appeared at a campaign event in the crucial early primary state of New Hampshire.

"Mr. Trump doesn't have a proven conservative record," Bush said. "People will vote for a proven conservative leader. Let's support someone where you don't have to guess where he stands."

Appearing on MSNBC's "Morning Joe," Trump responded to Bush's criticism on Thursday by comparing himself to conservative icon, and former president, Ronald Reagan, Politico noted.

Trump explained that Reagan "was (once) a Democrat, and really a Democrat with a liberal leaning, or a liberal bent and became a Republican and a pretty conservative Repub -- I wouldn't say the most by any stretch, but a pretty conservative Republican."

Reagan, who had served two terms as California governor before he was elected president in 1980, had famously quipped two years later that he, too, was to blame for the economic problems affecting the United States because "for many years, I was a Democrat."

Trump, meanwhile, also pointed to his upbringing in Queens and current residence in Manhattan to explain his lack of a conservative track record, Politico noted, telling the "Morning Joe" panel that virtually "everybody in Manhattan is a Democrat, everybody, and you sort of grow up in that atmosphere."

His convictions -- not labels -- are what truly matter, the Republican presidential frontrunner insisted.

"When you get down to it, I am a conservative person," Trump said. "I am by nature a somewhat conservative person. I never looked at putting a label on myself, because frankly putting a label on myself, it didn't matter -- I wasn't in politics. It was something that absolutely had no bearing on me."