Mitt Romney is calling Republican rival Rick Santorum "an economic lightweight" as the candidates make last-minute pitches to voters in Tuesday's Michigan and Arizona presidential primaries.

Romney says voters recognize that November's election will "come down to a race about the economy," and says Santorum is not the right candidate to challenge President Barack Obama.

Appearing on Fox News Channel, Romney said he doesn't think Republicans "want to nominate an economic lightweight to go up against the president, who also is an economic lightweight."

Romney also objected to Santorum's use of "robocalls" encouraging Democrats to vote in Michigan's Republican contest. Romney called it a "terrible, dirty trick."