JOHNSTON, Iowa (AP) — Newt Gingrich grinned as he pledged to dog President Barack Obama at every turn and from coast to coast next year if he's the Republican nominee."The White House will be my scheduler, and wherever the president goes, I will show up four hours later to respond to his speech," the GOP presidential candidate said wryly on a recent visit to Iowa.Seemingly in unison, the 500 Iowa Republicans crowded into the banquet hall rose from their seats applauding, for there he was — the tested antagonist that Republicans here have been craving to go toe to toe with the Democratic incumbent."We're looking for Ulysses S. Grant. And Newt Gingrich is the only one who has said we need to attack," said Craig Bergman, a Des Moines Republican who had been leaning toward Gingrich recently — and was hooked after last week's speech.If there's any one reason that may explain Gingrich's sharp rise in Iowa, where he now leads in polls, it's this: Republicans, in Iowa at least, are aching for an attack dog candidate in the effort to beat Obama.Indeed, prospective Republican caucusgoers, who are looking for a fighter prepared to go up against the well-funded, politically deft and oratorically gifted Obama, have gravitated to other GOP candidates not shy about lobbing verbal bombs at Obama — Michele Bachmann, Rick Perry and Herman Cain among them. But those candidates have either faded or dropped out. In Donald Trump's case, he never ran but his no-holds-barred criticism of the president helped him briefly rise to the top of national polls.Enter Gingrich.His pitched battles with Democratic President Bill Clinton while he was House speaker serve as an important reminder to GOP voters that he's challenged the opposition at its highest level. But, should he win the GOP nomination, he will have to do more than rally a frustrated GOP base; he will have to convince swing voters he can lead a worried nation.As Jim Dyke, a former Republican National Committee communication director now based in South Carolina, put it: "He's been a chief antagonist in the past, so that certainly gives him credibility. ... But we're not voting for chief antagonist. We're voting for president."First, however, the candidate must get through the GOP nomination race.And, less than a month until the leadoff Iowa caucuses, Gingrich's reputation as a bulldog is setting up a key stylistic contrast to former Massachusetts Gov. Mitt Romney, who is focusing largely on Obama's handling of the economy in his second bid for the GOP nomination.Compared with Romney, Gingrich seems more at home in the role of adversary. And he doesn't simply note his disagreements with Obama. He casts himself as the Democrat's philosophical opposite."He is an Alinsky radical," Gingrich told The Associated Press last week, calling Obama a disciple of Saul Alinsky, the late left-wing activist from Chicago. "And I am an American exceptionalist. He believes in fundamentally undermining the America we inherited. I believe in fundamentally rebuilding the America we inherited."A look at the past illustrates Gingrich's knack for confrontation.He was the engineer of the Republicans' 1994 House takeover. By 1995 and 1996, he was engaging in an epic battle with Clinton; the federal government shut down twice after the Democratic president and Republican-led Congress could not agree on a budget deal.Today, the 68-year-old Gingrich has not mellowed in his tendency for inviting sweeping confrontation, recently telling an audience of Texas conservatives, "I am convinced that if we do not decisively win the struggle over the nature of America, by the time (my grandchildren) are my age they will be in a secular atheist country, potentially one dominated by radical Islamists and with no understanding of what it once meant to be an American."He also is working to turn his own perceived weaknesses into points of contrast with Obama. The Republican, for example, is facing criticism for supporting legal status for longtime, law-abiding illegal immigrants with community ties. But he didn't hesitate to assail the Obama administration for suing South Carolina over an immigration law."Here's a simple way to think of it: President Obama sided with Mexico. I would side with South Carolina," he said last week in Charleston, S.C.Perhaps mindful that he can sometimes take his attacks too far, Gingrich is seeking to emphasize his softer side in his campaign advertisements. In his first TV commercial in Iowa, he promotes "working together" and "respecting one another" while making an upbeat call for unity.Even so, his provocation of Obama thrills partisan audiences — at least the one last week at the Polk County Republicans' annual fundraiser in Johnston.If he's the nominee, Gingrich said, he will invite Obama to debate seven times in the three-hour Lincoln-Douglas style and added, "How does a Columbia, Harvard law graduate, editor of the law review, greatest orator in the Democratic Party, look in the mirror and say he's afraid to stand on the same platform with a West Georgia College professor?"And with that playful taunt, Gingrich had his audience.