GRAND JUNCTION, Colo. — Mitt Romney is dispatching top surrogates to attack rival Rick Santorum over Santorum's past support for spending on pork-barrel projects.

Romney is the front-runner in the Republican presidential race and he's working to fend off an unexpected challenge from former Pennsylvania senator Santorum in the next states to vote.

Those are Colorado and Minnesota on Tuesday.

Santorum is countering the attacks and saying Romney should not be the GOP nominee. Santorum points to the health care overhaul Romney signed into law in Massachusetts when he was governor as one reason why not.