Grover Norquist of the Americans for Tax Reform group told The Washington Post there was too much uncertainty about attendance, with some candidates already opted-out.
In particular, Texas Gov. Rick Perry has yet to formally announce his intention to run, despite a rousing endorsement at last weekend's Republican Leadership Conference.
The debate was to be co-hosted by the tax reform group and the conservative Web site The Daily Caller.
An alternate date wasn't released.
Former Massachusetts Gov. Mitt Romney has said he wouldn't participate and former Utah Gov. Jon Huntsman was reportedly "unlikely to participate," the Post said.
Perry strategist Dave Carney told the newspaper the governor was still grappling with the logistics of running a federal campaign for the 2012 election.
"Can we build a financial infrastructure to raise the money without using so much of the governor's time that he can't campaign in the early states?" Carney said. "If you can do one without the other, it doesn't really matter -- you've got to be able to do them both. That's the real question, and we don't know the answer to that."
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