MANCHESTER, Iowa - More than $1 million in negative advertising - much of it bankrolled by Mitt Romney's allies - has eroded Newt Gingrich's standing in Iowa and thrown the Republican presidential race here wide open two weeks before the first votes.

The former House speaker's Iowa slide mirrors his newfound troubles nationally and has boosted Romney's confidence while fueling talk that libertarian-leaning Texas Rep. Ron Paul could pull off a win Jan. 3 in the leadoff caucus state.

"It's very disappointing to see so many of my friends who are running put out such negative junk," Gingrich said Monday as he arrived in Davenport, poking at his opponents even as he insisted he was running an upbeat campaign. "I really wish they would have the courage to be positive."

Despite his chiding, attacks against him are all but certain to continue. For one, the Restore Our Future political action committee, made up of former Romney staffers from his failed 2008 bid, plans to spend $1.4 million more over the next two weeks, including a new ad beginning Tuesday that is expected to be aimed at Gingrich.

Aides for several campaigns competing against Gingrich as well as outside independent groups aligned with the candidates say their internal polls find he has fallen over the last week from the top slot in Iowa. And a national Gallup poll released Monday found Gingrich's support plummeting: He had the backing of 26 percent of Republican voters nationally, down from 37 percent on Dec. 8. Romney's support was largely unchanged at 24 percent.

Gingrich's weakened position follows a barrage of advertising, primarily financed by so-called super PACs, that cast him as a longtime Washington power broker.

The results in Iowa will shape the rest of the state-by-state march to the GOP nomination. Gingrich rushed back to the state Monday after a three-day absence for several days of campaigning.

He told about 200 people at a security company in Davenport that he would begin a 44-stop Jobs and Prosperity tour before the caucuses and use those events to answer any charges put out there.

Gingrich also redoubled his appeals to conservatives with sharp criticism of the judiciary, saying he would have the Justice Department instruct the U.S. Marshals Service to arrest judges who ignore subpoenas to testify in Congress about their decisions. And he tried anew to end accusations he lobbied on behalf of troubled Freddie Mac or other organizations.

"We should have had a much more coherent answer," he said about assertions that he earned a windfall from the federally backed mortgage giant. He offered his latest explanation, saying that his consulting firm was hired over a six-year period for strategic advice and that he earned about $35,000 a year - "less than I got per speech." Altogether, Gingrich's firm earned about $1.6 million from Freddie Mac.

Romney was largely laying low, increasingly expressing optimism as he revels in a series of endorsements from establishment GOP figures such as Bob Dole, the 1996 GOP nominee, early-state leaders such as South Carolina Gov. Nikki Haley, and newspapers including the Des Moines Register.

Paul, who has built arguably the largest get-out-the-vote organization in Iowa and has steadily been inching up in Iowa polls, spent the day in New Hampshire before returning to Iowa for a packed schedule later in the week. He has been on the air here with ads assailing Gingrich.