Billionaire George Soros is weighing in on the U.S. healthcare battle, pledging $5 million to a coalition of left-wing groups seeking the so-called “public option” — socialized medicine.

But activists are disappointed that the group Soros quietly contributed to — Health Care For America Now, the leading coalition of pro-reform groups, unions and providers — hasn’t been nearly as aggressive in pushing Democrats toward the public option.

“The Soros pledge is noteworthy, because both sides will seize on it,” reports The Plum Line blog. “The right will say it shows the real astroturfing is coming from the pro-reform side — billionaire bogeyman Soros is bankrolling this fight!

“The left will cite the donation to demand that HCAN show real results,” the blog continued. “Some on the left, such as blogger Jane Hamsher, have been asking why heavily bankrolled HCAN hasn’t been able to secure more commitments from Dem members of Congress to stand firm behind a public option.”