By Mark Felsenthal
OTTUMWA, Iowa (Reuters) - Slipping in polls under an onslaught of attacks, Republican presidential contender Newt Gingrich promises to wage a sharper and more aggressive campaign in the final two weeks before Iowa kicks off the party's nominating race.
Gingrich says he will be quicker to respond to attacks and more agile in getting his message across to voters who may have lost faith in him after rivals launched a withering barrage of criticism of his past as a Washington insider.
A 44-stop bus tour in the run-up to the January 3 Iowa caucus, daily calls with supporters to respond to new criticisms and another round of television ads will be cornerstones of the response by Gingrich, who will focus most of his efforts in Iowa for the next two weeks.
"I think a certain amount of drop in polls was to be expected given the amount of negative campaigning that has been going on," said Linda Upmeyer, the Iowa House Republican leader and the state chairwoman of Gingrich's campaign.
"But there are a lot of people out there who have not made up their minds yet," she said. "The challenge is to make sure that people have accurate information, and to get them the facts as quickly as we can."
The question for Gingrich will be whether he has enough time to rebound in the two weeks left to campaign in Iowa or if he has begun a political free-fall that will be hard to stop.
"The problem is he has probably waited too long," said Craig Robinson, former political director for the Iowa Republican Party. "This is the frantic finish. The clock might run out on him."
Gingrich, the former speaker of the U.S. House of Representatives, has seen his national lead in polls over rival Mitt Romney disappear in the last week, with new CNN and Washington Post/ABC polls showing him falling back into a tie with Romney.
In Iowa, he has fallen behind rival Ron Paul as airwaves and mailboxes fill with ads from his rivals and from outside groups attacking him on his past as a Washington power-broker and on the $1.6 million he made working for mortgage giant Freddie Mac.
Gingrich said he was disappointed in all the "negative junk" and renewed his promise not to attack fellow Republicans. But he admits he was slow to counter the charges about his work for Freddie Mac, which many Republicans blame for the housing collapse.
"What we should have done immediately after the first time this came up is we should have stopped, pooled together everything, and I should have had a much more coherent answer," Gingrich said on Monday at a small business in Davenport, Iowa, promising he would answer their concerns point by point.
The charges that Gingrich profited from his former position as House speaker to lobby Freddie Mac show no sign of going away. It was raised with Gingrich by another voter on Monday.
Upmeyer said it took a while for the campaign to recognize the criticisms were taking hold so strongly. "That was clearly one that voters wanted more information on," she said.
She said the televised town hall meetings with supporters had been an effective format for Gingrich in the past, and the bus tour would give him an opportunity to meet voters face-to-face and answer any questions they have.
"Long before the caucuses, you'll know every answer to every single negative ad," Gingrich told a crowd of about 300 on Monday night at Level Ten, an apparel business in the Iowa town of Hiawatha.
The attacks on Gingrich have primarily come from Paul and from a pro-Romney political action committee, and Gingrich on Tuesday called on Romney to ask the committee to take the ads down.
"This PAC was created by his former staff and funded by his personal friends," he told reporters. "He can demand that every ad be positive. I don't object to being outspent, I object to lies."
Voters said they wanted to see Gingrich respond more forcefully to the attack ads.
"The elephant in the room for Newt is he has so much excess baggage, and that stuff gets hammered home," said Jerry Marietta, 62, from Davenport.
"If you believe any of the negative ads, you think to yourself, 'Oh my gosh,'" Marietta said. "I think he has to articulate a really strong message to counter the negative ads."
Gingrich is the latest of a series of Republican contenders to rise briefly to the top in the race to find a challenger to President Barack Obama in 2012, only to quickly fall back.
Congresswoman Michele Bachmann, Texas Governor Rick Perry and businessman Herman Cain all had their turns in the limelight only to wilt, but Gingrich has shown an ability to calmly handle tough questioning in recent debates.
"There are people who really, really like Newt Gingrich," Robinson said. "He just needs to get in front of as many people as he can. I think people would really, really like that."
Gingrich will receive endorsements on Wednesday from the speaker of the Iowa State House, Kraig Paulsen, and the speaker of the New Hampshire State House, Bill O'Brien.
But he lost the endorsement on Tuesday of influential religious conservative Bob Vander Plaats, head of the Christian group The Family Leader, who backed former U.S. Senator Rick Santorum.
The Vander Plaats endorsement was a blow to Gingrich, who had helped the Iowan in last year's drive to remove three Iowa Supreme Court judges who had backed same-sex marriages.
While Upmeyer said Gingrich's closing campaign message will remain positive, he has made clear he will draw a contrast between his approach and the style adopted by his colleagues.
"My only request to the people of Iowa is when you get ready to vote in two weeks, ask yourself, 'Do you really want to reward politics as usual, negativity as usual, attack as usual, consultant as usual, fundraising from Wall Street millionaires as usual?'" Gingrich said in Hiawatha.
"Or do you want to vote for the only person who has consistently, steadily been positive for the entire campaign?"
(Writing and additional reporting by John Whitesides; Editing by Xavier Briand)