Tim Pawlenty is running hard again — this time to be Mitt Romney’s running mate. He is trying to prove his loyalty and his effectiveness by doing Romney’s dirty work, an important quality for a veep.

Some believe Rick Santorum is poised to do well today in Pawlenty’s home state of Minnesota, and the Romney team has sent the former governor on a seek-and-destroy mission. Pawlenty, who, in his brief run for the nomination was reluctant to attack, has developed a taste for the jugular, calling Santorum unqualified to be president.

Pawlenty is walking a fine line in his try-out for a spot on the ticket. Presidential nominees often need and want an attack dog. But if the dog snarls and bites too much, it will stay in the pound.