Romney has emerged as the GOP's leading candidate, but his critics are already telegraphing their eagerness to link him with President Barack Obama's health care overhaul. Conservatives loathe the Democrats' plan that mandates all Americans have health insurance and was modeled on a Massachusetts law Romney signed into law.
Former Minnesota Gov. Tim Pawlenty is trying to pitch himself as an alternative to Romney, who lost the nomination four years ago to John McCain.
"I strongly oppose the individual mandate at any level," Pawlenty said on Sunday, previewing an expected line of criticism toward Romney. "I think it's a dramatic overreach."
Also expected at the debate are former House Speaker Newt Gingrich, Rep. Ron Paul of Texas, former Sen. Rick Santorum of Pennsylvania, Rep. Michele Bachmann of Minnesota and businessman Herman Cain of Georgia.
Bachmann is not yet a formal candidate, but the tea party favorite is expected to join the field soon.
Obama's handling of the economy, health care and wars in Afghanistan, Iraq and Libya are expected to draw fire from the Republican field.
The Associated Press contributed to this report.