POLITICO's Juana Summers reports from Colorado Springs:

Rick Santorum told Colorado Springs supporters Tuesday that Mitt Romney and Newt Gingrich would be ineffective challengers to President Barack Obama in the general election because of their views on cap and trade.

"Let's go to cap and trade. Governor Romney proudly announced that they were the first state, Massachusetts, to put a cap on CO2 emissions in the state of Massachusetts," Santorum told more than 100 supporters here.

Santorum also dinged Gingrich for filming a climate change ad with Nancy Pelosi, something Gingrich - as recently as yesterday - has admitted was a mistake. Both Gingrich and Santorum appeared at a Colorado energy conference on Monday.

"Speaker Gingrich has supported cap and trade for more than a dozen years. Now, he wants business incentives to go along with cap and trade, but he supported cap and trade, and sat on the couch with Nancy Pelosi and said that global warming had to be addressed by Congress," Santorum said. "Who is he or who's Governor Romney to be able to go after President Obama? I've never supported even the hoax of global warming."

Romney and Gingrich, Santorum said, "bought into the science of man-made global warming, and they bought into the remedy, both of which are bogus."