Jon Huntsman sent an open letter to John Sununu (R) arguing the former New Hampshire governor made a mistake in endorsing former Massachusetts Gov. Mitt Romney (R).

Huntsman argued Romney isn't a real conservative.

"Over the course of this campaign you have made it abundantly clear that you would endorse a conservative governor — a laudable criterion," Huntsman wrote in the letter. "However, I am surprised that you believe Mitt Romney meets that threshold."

Huntsman listed instances where he took the more conservative stance than Romney, such as on taxes, the NRA and abortion.

"While Mitt Romney opposed the Bush tax cuts and raised taxes and fees by $750 million in Massachusetts, I signed the largest tax cut in Utah history, which helped our state lead the nation in job growth," Huntsman wrote. "While Mitt Romney was once ardently pro-choice — stating in 1994 that 'abortions should be safe and legal' — I am proud to be a lifelong defender of the sanctity of life."

Huntsman stressed that Romney actually has cultivated fairly liberal policy stances.

"When you look at his past statements, positions and voting record, the idea that Mitt Romney is a principled conservative is an impossible conclusion," Huntsman continued. "It is more than his one term dealing with a liberal legislature; it’s a lifetime and record of inconsistent and liberal positions."

Huntsman's letter comes a day after Romney announced that Sununu had decided to endorse the former Massachusetts governor. Sununu's endorsement is of particular interest to Huntsman's campaign, which has increasingly staked its future in the 2012 presidential election on New Hampshire. Huntsman has been hoping to dominate the state's early Republican primary and use that as a springboard in his campaign to win successive Republican state primaries.

"Mitt Romney and I have two very different records and visions for America’s future, which I look forward to sharing with voters in New Hampshire and across the nation," Huntsman wrote. "I even look forward to sitting down with you at some point if you ever care to learn the truth."

Later on Monday on Fox News, Huntsman said he wanted to point out his conservative record as governor of Utah compared to Romney's as governor of Massachussetts.

"The good Governor can do whatever he wants. I just wanted to set the record straight in terms of him jumping behind Governor Romney saying that he was looking for a conservative," Huntsman said. "I say there’s not a whole lot conservative in terms of where Governor Romney has been over the years and I wanted to point out where I’ve been in terms of a conservative Governor of the state of Utah."

—This story was updated at 1:24 p.m.

Read the letter below:

10.23.2011 Letter to Gov. Sununu