Sen. Mark Begich, D-Alaska, thinks voters should not conclude, based from the fact that he signed a 2010 letter calling for government to "mak[e] polluters pay a price on greenhouse gas" that he supports a carbon tax.

"I didn't agree with every piece of that letter -- people think when you sign those letters, you are 100 percent," Begich told the Anchorage Daily News. "My objective with that letter was to make sure we did not lose the debate and the discussion of what we needed to be doing around climate change."

The "price" was the top priority of the group of senators who wrote the letter to Majority Leader Harry Reid, D-Nev., on July 16, 2010, just days before Reid gave up on trying to pass cap-and-trade legislation during that election year.

“First, we believe the scale of this challenge dictates the need for a comprehensive solution that includes making polluters pay through a price on greenhouse gas emissions," the senators wrote.

Americans for Prosperity cites that letter as evidence for a campaign ad charging Begich with supporting a carbon tax, something that Begich denies ever supporting.

"The letter to Reid said that energy policy should be aimed at putting some sort of price on greenhouse gas emissions, without specifically saying it should be a carbon tax," Politifact said in deciding to rate the ad "mostly false."

Begich is one of the most vulnerable Senate Democrats, as Mitt Romney beat President Obama in Alaska 55 percent to 41 percent in the 2012 presidential election.