WASHINGTON -- Hours before Democrats passed a historic cloture motion to advance the health care bill, one of its critical backers announced that his vote remains conditional on a set of demands.

It had better not tax the rich, Sen. Ben Nelson (D-NE) said Sunday in an appearance on CNN's State of the Union, or he could withdraw his support.

CNN paraphrased Nelson as saying, "Imposing a tax on wealthier Americans, a funding mechanism employed in the House health care reform bill, could also cause him to withhold his vote for the final version of the bill."

A surtax on the wealthiest Americans is one method Democrats have considered to pay for the bill's $871 billion price tag over 10 years. But in the series of cloture votes ahead, Democrats can't afford to lose the support of a single member, so Nelson's demand is likely to be be taken seriously.

"I'm an independent-minded sort of person," Nelson said. "And I think that's demonstrated. I don't take my marching orders from a party or a group or any other entity."

The legislation currently contains a 10 percent excise tax on indoor tanning services, according to the CBO, which was added in lieu of a 5 percent tax on cosmetic surgery.

Nelson did not address either of these specifically, but they could fall under the purview of a tax on rich Americans.

A November poll conducted by the Associated Press found that Americans are largely supportive of taxing the wealthiest to pay for health care reform.

The House health care bill, which passed in November, slaps a 5.4 percent income tax surcharge on individuals making more than $500,000 annually, as well as families making more than $1 million.

Nelson also reiterated his earlier stance that restrictions on abortions must be strong, or else he could back out. This position led to a series of 11th hour negotiations with the Democratic leadership that ultimately won his approval.

Finally, Nelson insisted there be no public option in the final bill -- which was scrapped last week due to recalcitrant senators -- or Democrats could lose his vote.

"I suppose putting in a public option would do it," Nelson said on CNN.