GREEN BAY, Wis. – Another victory in hand but still badly trailing rival Mitt Romney, Republican presidential hopeful Rick Santorum vowed to continue his campaign despite a steep climb to the nomination.

Santorum, buoyed by Saturday’s win in Louisiana’s primary, urged supporters to stick with him even as much of the GOP establishment has coalesced around Romney’s increasingly inevitable coronation.

The former Pennsylvania senator seemed unwilling to acknowledge it would take a dramatic change in momentum to deny Romney his turn as the GOP nominee.

“Even though a lot of folks are saying this race is over, the people in Louisiana said, ‘No, it’s not.’ They still want to see someone who they can trust, someone who’s not running an Etch a Sketch campaign, but one who has their principles written on their heart, not on an erasable tablet,” Santorum said Sunday on CBS’ “Face the Nation.”

Romney remains far ahead with 568 delegates to Santorum’s 273, according to an Associated Press tally. Newt Gingrich follows with 135 and Ron Paul has 50.

Santorum was campaigning Sunday in Wisconsin, which holds its primary on April 3. Aides look ahead to the state as a bright spot, as well as Pennsylvania, the delegate-rich state Santorum represented in Congress.

But Romney’s campaign is airing TV ads in the state, and his super PAC allies have plowed more than $2 million into TV advertising here. A crush of advertising — mostly negative — eroded Santorum’s strength in states such as Michigan, Ohio and Illinois as he simply couldn’t keep pace.