An InsiderAdvantage poll finds Romney with the support of 23 percent of likely caucus-goers, ahead of Paul at 22 percent.
Santorum’s events have swelled past capacity as both voters and media have flocked to see the former Pennsylvania senator speak.
GOP hopeful Newt Gingrich downplayed his expectations for the Iowa caucuses on Monday.
There is growing belief in Santorum’s campaign that Tuesday's caucuses are coming down to a fight against Romney.
"I think I'm getting a real strong send-off," Romney said of his recent efforts to court Iowa caucus-goers.
"Mitt Romney learned a lesson, that you can't ignore Iowa too long," said Branstad of Santorum's surge.
Rep. Michele Bachmann said concerns conservatives wouldn’t rally behind one candidate was a “big issue” ahead of the Iowa caucuses.
A new report calls the ever-changing nomination fight the "most volatile for the GOP since the advent of polling."
The GOP hopefuls are criss-crossing the state Monday and revealing different strategies as they look for a late edge.
Santorum’s aides insist they are well-placed to capitalize on the recent groundswell of enthusiasm.