Santorum has more delegates in the five states than any other candidate, according to The AP tally: 73, compared to 62 for Romney, 15 for Paul and none for Gingrich.
Most of the campaigns argue they will outperform AP's projections, and the battle in some states is fierce.
In Missouri, a local Republican caucus near St. Louis was shut down without a vote after it got so rowdy that extra police were summoned to help. In Wyoming, Romney supporters successfully challenged a single Santorum delegate in Park County, getting local party officials to switch the delegate to Romney more than a week after the convention.
In Minnesota, Paul's supporters showed up at the GOP convention in Carver County lugging computers and printers so they could track their delegates and tell supporters whom to vote for.
"First time I'd seen it," said Steve Nielsen, the Carver County GOP chairman.
Despite not winning a state, Paul's supporters have been active in all the caucus states.
John Tate, Paul's campaign manager, said Paul's supporters have sometimes played kingmaker, cutting deals in counties where the original caucus vote was evenly split, resulting in a divided county convention.
In some places, they have been approached by Romney supporters looking to block Santorum's delegates. In others, they have been approached by Santorum supporters looking to block Romney.
Said Tate: "Unless you have an outright majority, cutting deals happens."
He said Santorum's campaign has shown signs of life in the caucus states, but he questioned whether it would be enough to win many delegates from Romney.
"I think his campaign is starting to figure it out," Tate said of Santorum. "But in a lot of these states, I don't see the organization that we have or that Romney has."
Associated Press writers Brian Bakst in St. Paul, Minn., and David A. Lieb in Town and Country, Mo., contributed to this report.
- Super Bowl throwback: 11 heart surgeries...
- Survivors pulled out from building 2 days...
- Debate Takeaways: Rubio shaken, Trump not...
- North Korea's new satellite flew over Super...
- Rescuers in Taiwan pull out survivors from...
- 10 Things to Know for Today: New Hampshire,...
- General Mills’ Betty Crocker, Pillsbury...
- Christie, others hit final stretch in New...
- US adds just 151k jobs in January; 4.9... 29
- Clinton seeks to cut into Sanders' New... 9
- 5 things to know about the Super Bowl... 9
- Clinton's New Hampshire challenge:... 6
- US productivity fell at sharp 3 percent... 5
- Democratic race down to just two:... 5
- Debate Takeaways: Rubio shaken, Trump... 5
- For the Clintons, New Hampshire is the... 5