Charlie Riedel/AP Photo

To conservatives, the biggest strike against Mitt Romney is the health care plan he put in place in Massachusetts, but Newt Gingrich lavished praise on Romney's plan after it was passed in 2006.

"We agree entirely with Governor Romney and Massachusetts legislators that our goal should be 100 percent insurance coverage for all Americans," Gingrich wrote in 2006.

And, Gingrich wrote, the key to achieving that goal was doing what Romney did in Massachusetts: Requiring everybody who could afford it to buy health insurance. In fact, Gingrich makes an impassioned case for the so-called individual mandate - which is also at the center of President Obama's health plan - on conservative grounds.

"We also believe strongly that personal responsibility is vital to creating a 21st Century Intelligent Health System," Gingrich wrote in the memo which was found on an old Gingrich website by the Wall Street Journal's Brody Mullins and Janet Adamy.

"Individuals who can afford to purchase health insurance and simply choose not to place an unnecessary burden on a system that is on the verge of collapse; these free-riders undermine the entire health system by placing the onus of responsibility on taxpayers."

Read the memo.

Gingrich had some criticisms of the Massachusetts plan - including what he called the state's over-regulation of health insurers - but overall, he loved it.

Newt: "Massachusetts leaders are to be commended for this bipartisan proposal to tackle the enormous challenge of finding real solutions for creating a sustainable health system."