Republican presidential candidate John McCain's signal that he may be open to a higher payroll tax for Social Security, despite previous vows not to raise taxes of any kind, is drawing sharp rebukes from conservatives.

McCain's shift has come in stages, catching some Republicans by surprise. Speaking with reporters on his campaign bus on July 9, he cited a need to shore up Social Security. "I cannot tell you what I would do, except to put everything on the table," he said.

He went a step farther Sunday on ABC's "This Week," in response to a question about payroll tax increases.

"There is nothing that's off the table. I have my positions, and I'll articulate them. But nothing's off the table," McCain said. "I don't want tax increases. But that doesn't mean that anything is off the table."

That comment drew a strong response this week from the Club for Growth, a Washington anti-tax group. McCain's comments, the group said in a letter to the Arizona senator, are "shocking because you have been adamant in your opposition to raising taxes under any circumstances."

Indeed, McCain frequently has promised not to raise taxes.

At a July 7 town-hall meeting in Denver, he said voters faced a stark choice between him and Democrat Barack Obama.