"Thom is a conservative who has been solving problems in North Carolina as Speaker of the House and I am confident he will do the same in Washington," Romney said in an email sent out to supporters by the Tillis campaign.
Paul endorsed Tillis in October and is campaigning for him Monday at rally in Charlotte.
Tillis is the frontrunner in the primary and his concern going into election night on Tuesday is whether he can get the 40 percent support of primary voters he needs to avoid a runoff. Recent polling has shown Tillis getting that support. But if he is forced into a runoff, Tillis is likely to face Brannon.
If a runoff is triggered it will be held on July 15.