The next primaries are April 3 in Wisconsin, Maryland and the District of Columbia. Santorum failed to make the ballot in the District of Columbia. His best shot is in Wisconsin, where he led in polls taken in February. But a survey completed after Romney's thumping win in neighboring Illinois put the former Massachusetts governor comfortably ahead.
Then the campaign takes a hiatus until April 24, when New York, Pennsylvania, Connecticut, Rhode Island and Delaware hold primaries.
Santorum is investing heavily in winning his home state, which he represented for four years in the U.S. House and 12 years in the Senate. But he failed to win his bid for a third term in the Senate, losing by 18 points.
At a conservative conference Saturday in Milwaukee, Santorum continued the scathing assault on Romney that he launched in Louisiana.
He held up an Etch A Sketch and shook it, evoking a Romney aide's statement that his boss could reset his positions in the general election campaign.
"When people vote for president, they want to vote for someone they trust," he said, "someone who's authentic, someone who won't say anything to get elected, but will stand up and do what's right for the people of this country."