Sen. Ben Nelson (D-Neb.) suggested on Thursday that he thinks a flat tax might be a "great idea" if implemented correctly.

Nelson, a centrist who sometimes breaks with Democrats in certain key votes, expressed worries that key elements of such a tax could make it difficult to install.

"It's  great idea, it's hard to implement," Nelson said on KFAB radio in Nebraska.

The flat tax is an idea that has long been pushed by economic conservatives -- most prominently former House Majority Leader and Tea Party movement leader Dick Armey -- that would see all income tax brackets replaced with a single income tax rate applied to all earners. Supporters of the flat tax argue that if virtually no deductions were allowed from a flat tax, many of the loopholes exploited by high earners and businesses would be closed, and the government would raise adequate money while simplifying the system.

Nelson said he worried that the lack of deductions would be a major turn-off for voters.

"You'll have to think about charitable contributions -- are they included. Mortgage deductions?" he asked. "Once you start dealing with those kinds of issues, the flat tax isn't flat anymore."

Nelson is one of the Democrats who's joined with Republicans at times on tax issues. Like many Republicans, he favors extending the tax cuts set to expire at the end of the year for all income brackets.

He hit GOP leaders for "hold[ing] hostage" most tax cuts for middle class earners by insisting that a vote to extend the highest-end tax cuts can't be held separately.

"I just don't think it's right to hold hostage to get the upper income, but I want all of them," he said. "I'm not sure the other side's negotiating in good faith, either."