Rivals heaped criticism on surprise front-runner Rick Santorum in a debate among U.S. Republican presidential candidates on Wednesday, hoping to stall his surge at a pivotal period in the 2012 campaign.

Mitt Romney and libertarian congressman Ron Paul tried to raise doubts about Santorum and questioned his fiscal conservative credentials based on his time in the U.S. Congress when he was an easy backer of government spending projects deemed wasteful by critics.

Paul, asked by CNN moderator John King why he had launched a campaign ad that labelled Santorum a fake, responded: Because he's a fake.

Romney, scrambling to make up for a deficit in the polls to Santorum, put the former U.S. senator from Pennsylvania on the defensive for backing a much-derided $400 million bridge to nowhere project in Alaska that was eventually abandoned.

Santorum fired back that Romney, as chief organizer of the 2002 Winter Olympics in Salt Lake City, had sought similar government spending items known as earmarks and often deemed wasteful spending.

When I was fighting for the Olympics, you were fighting for the 'bridge to nowhere,' Romney told Santorum.

You don't know what you're talking about, Santorum snapped back. He insisted that earmarks do enjoy public scrutiny and can be useful, although there has been a move among congressional Republicans to ban them.

Santorum needs to build on his momentum going into the Arizona and Michigan primaries on February 28 and pave the way for Super Tuesday on March 6.

Santorum and Romney are in a close race in Michigan, according to opinion polls, with most recent surveys showing the two divided by 4 percentage points or fewer. A victory in Michigan is critical for Romney as he needs to prove he can win in the state where he was born.

An NBC/Marist poll on Wednesday had Romney at 37 percent to Santorum at 35 percent among likely primary voters in Michigan, a statistical dead heat.

The poll gave Romney a lead of 42 percent to 27 percent over Santorum in Arizona. A Time/CNN poll on Tuesday found Romney ahead by 36 percent to 32 percent there.

A Quinnipiac University poll also released on Wednesday showed Santorum up 9 percentage points nationally against his Republican rivals, at 35 percent compared to 26 percent for Romney. It showed Gingrich with 14 percent and Paul with 11 percent.

(Additional reporting by Patricia Zengerle; Editing by Alistair Bell and Christopher Wilson)