He says they need to stay on the sidelines if they can't get primary nods.
The former Massachusetts governor is particularly concerned that a third party effort will kill Republican hopes in 2012 (much like Ralph Nader is believed to have done to the Democrats in 2000).
"If there is a conservative candidate that runs in the general election, then obviously, divide and fail is the result," Romney said.
"Hopefully Tea Party candidates will run in respective primaries and they will either win or lose. And if they win, they will go into the general. If they lose, they won't, and they will get behind the more conservative of the two finalists."
Romney said "dividing our conservative effort in the general elections" would "basically hand the country to Barack Obama and Nancy Pelosi and Harry Reid, and that would be very sad indeed."