Are Donald Trump and Ben Carson closet Democrats?

Both Donald Trump and Ben Carson have registered as Democrats in the past, and both have a history of liberal positions. 

Donald Trump's record is so liberal some political observers have openly wondered if he is a Democratic plant. Ben Carson, who has praised liberal icon Elizabeth Warren and once backed universal health care, says he used to be a "flaming liberal."

In past campaigns, that could have sunk a candidate.

Not in the 2016 race. Today, the unlikely pair is leading the Republican presidential race both in national polls and in early nominating states like Iowa, surpassing tried-and-true conservatives like former Arkansas Gov. Mike Huckabee and Wisconsin Gov. Scott Walker.

It's a surprising twist in a race that gives the appearance of upending many of the old rules. The success of Mr. Trump and Dr. Carson – two of the least politically experienced candidates in the race – is an indication that so far anti-establishment fervor has eclipsed traditional benchmarks like public service and ideological purity on social and religious issues.

Trump's liberal past is no secret, yet it hasn't appeared to hurt the billionaire businessman, who has claimed top spots in almost every poll since he declared his candidacy.

In the first GOP debate, Trump was pressed to explain his past support of liberal policies like single payer health care, for which he was ridiculed by rival Rand Paul.

"News flash, the Republican Party's been fighting against a single-payer system for a decade," the senator from Kentucky said. "So I think you're on the wrong side."

In the past, the real estate mogul has backed an assault weapons ban, argued for tax hikes on the wealthy, and registered as a Democrat.

It was enough for rival Jeb Bush, who has arguably suffered the most from Trump's rise, to release an attack ad Tuesday shredding Trump's conservative credentials.

"I probably identify more as a Democrat," Trump says in archive footage in the ad titled "Liberal things that Trump says."

The ad includes clips of Trump backing state-run health care, praising Democratic front-runner Hillary Clinton as a "terrific woman," and declaring, "I'm very pro-choice."

Its purpose is clear. "This is a pretty clean hit on Trump all wrapped up in an easy way for people to watch and share," Craig Robinson, editor-in-chief of the Iowa Republican, told CNN.

"While it might not help Jeb Bush much, I do think it starts a narrative where people will start to look back and say, 'Who is this guy who is leading the field?' " he added.

For Carson, the former pediatric neurosurgeon and former "flaming liberal," the situation is slightly different.

While he was once a self-described "pretty left-wing Democrat" who argued against deportations, praised Elizabeth Warren, backed alternative fuels, and embraced universal healthcare, he has since worked hard to build credibility on conservative issues.

He launched himself onto the national stage when he famously lambasted President Obama on political correctness and the national debt at the National Prayer Breakfast in 2013. He's also become a regular on the talk radio circuit and in conservative publishing and newspapers.

In recent years, he has staked a strong right-leaning position on a number of social issues, like abortion and homosexuality.

In races past, a Trump or Carson may have fallen victim to flip-flopping smears, as happened to Democratic presidential candidate John Kerry circa 2004.

Not yet in the 2016 race, where the front-runners need only turn to a conservative hero to explain their change of heart.

"It's easy to explain," Trump told Sean Hannity a few weeks ago. "Ronald Reagan was a Democrat, and he was sort of liberal."