"As a protest, he's a reasonable candidate, [but] I don't see how you can engage Ron Paul as the nominee," the former House speaker said of the Texas congressman, ahead of the Iowa caucuses Tuesday -- which open the 2012 Republican nominating contest.
Paul's "views are totally outside the mainstream of virtually every decent American," Gingrich told CNN.
He said he could not support Paul if Paul won the Republican nomination.
"I think the choice of Ron Paul or Barack Obama would be a very bad choice for America," Gingrich told the network.
Gingrich, who has fallen in polls after leading a month ago, and the other Republican hopefuls sharpened their attacks on Paul in an effort to keep his support from growing among voters frustrated with government who may choose to send a message to Washington by supporting him in the caucuses, The New York Times said.
Former Sen. Rick Santorum of Pennsylvania told voters in western Iowa, "The things that most Iowans like about Ron Paul -- his economic ideas -- are the things he's least likely to be able to accomplish.
"He's been in Congress for 20 years and hasn't passed a bill," Santorum said.
He warned Republicans to be wary of Paul's isolationist foreign-policy views.
As commander in chief, Paul "can pull all our troops home," Santorum said. "He can shut down our bases in Germany. He can shut down the bases in Japan. He can pull our fleets back. You think the rest of the world is going to say, 'Oh, that's great?'"
Paul was to resume his campaigning in Iowa Wednesday.
Former Massachusetts Gov. Mitt Romney arrived in the state Tuesday night, focusing his rhetoric on Obama: "Mr. President, you have now had your moment. We have seen the results. And now, Mr. President, this is our time."
Romney, who began his day in New Hampshire, offered no predictions about the caucuses, but said before flying to Iowa, "I think I'm going to get the nomination if we do our job right."
Texas Gov. Rick Perry -- who changed the slogan on his campaign bus to "Faith, Jobs, Freedom" from "Get America Working Again" -- focused Tuesday on making his case to religious voters.
At stops in Council Bluffs and Clarinda, Iowa, Perry's speeches included quotes from the Bible and promises to protect "innocent life."
"When the Lord asked the prophet Isaiah, he said, 'Whom shall I send and who will go for us?' Isaiah said, 'Here am I, send me,'" Perry said.
"This is your country. Taking her back is your calling. Join me in this mission," the Times quoted him as saying in Council Bluffs, a city on the east bank of the Missouri River across from Omaha, Neb.
Perry said he would shake up Washington with his call for a part-time Congress.
Rep. Michele Bachmann of Minnesota accused her rivals of being "confused" about abortion and same-sex marriage.
"Mitt Romney has defended gay marriage and even signed marriage licenses for same-sex couples, and Ron Paul doesn't believe the government should protect the institution of marriage," Bachmann said in Council Bluffs.
"I have a record of defending life, marriage and the family, and I'll protect them as president of the United States," she said.
With the candidates in a final push to win over undecided voters, the Occupy Des Moines movement announced Tuesday night more than 100 people would protest at the presidential candidates' offices.
Thirty people each said they would protest at the campaign headquarters of Obama and Romney, 18 people planned to protest at Paul's campaign offices, 10 each said they would protest at the Perry and Gingrich offices, seven planned to protest at Bachmann's and one person planned to be at Santorum's, The Des Moines Register reported.