MACKINAC ISLAND, Mich. – Gov. Rick Perry and his aides in Texas have spent hours studying old footage and records of Mitt Romney, stretching back nearly two decades, building a list of issues on which they believe Romney has waffled or wavered, seeking to brand him as inauthentic.

Romney's team is honing plans for an attack on Perry's readiness to be president and commander in chief. They intend to press Perry on foreign policy, demand that he produce a national jobs plan and relentlessly pursue the case that Perry is out of step with his party on how to address illegal immigration.

On both sides, well-financed allies are preparing nominally independent efforts on behalf of both men, from attack ads to get-out-the-vote efforts.

The lines of argument are hardening as the party's leading presidential candidates, who delivered speeches here Saturday, continued to dig in for what Republicans believe could be a long and bitter fight for the nomination. That battle extends into the spring as new rules allow contenders to pick up delegates even in states where they lose.

After three debates that have shaped the race for now into a Perry-Romney contest but also highlighted the imperfections and political vulnerabilities of both men, the campaign is now entering a new phase. The candidates have less than a week to make their pitch to donors before the third quarter closes Friday, a critical point that could further narrow the field.

The animosity between Perry and Romney has deepened as they compete for contributors, endorsements and, ultimately, the soul of the party. But the prospect of a two-man contest has only intensified the thirst for more options, including new calls for Gov. Chris Christie of New Jersey to reconsider candidacy.

For Perry, the glow of his arrival to the race has given way to the daily rigors of campaigning, which his advisers say has been more difficult than he expected. He has generated enthusiasm among many grass-roots conservatives and posted strong showings in many early polls.

But his shaky debate performance Thursday night in Florida underscored concerns among establishment Republicans and big donors about his electability and his skills as a candidate on a national stage – and the difficulty he has had planting serious doubts about Romney. He also finished a distant second place in a weekend Florida straw poll.

"Yep, there may be slicker candidates and there may be smoother debaters," Perry said Saturday, drawing modest laughter from a luncheon crowd here. "But I know what I believe in, and I'm going to stand on that belief every day."

Perry's candidacy has forced Romney to abandon his quiet front-runner strategy and plunge into the battle. He invokes his rival's name nearly everywhere he goes now, using Perry's record as governor and his pattern of outspoken statements to cast him as inconsistent in his conservative principles and less likely to defeat President Barack Obama.

"I'm going to be a Republican candidate who can win," Romney told an audience last week, suggesting that he has greater crossover appeal to women and independent voters. "To beat the president, you've got to have credibility."

The early political strategies of both candidates are becoming more clear, with Perry waging a campaign in all of the early-voting states – the caucuses in Iowa and Nevada, along with the primaries in New Hampshire and South Carolina – and Romney concentrating on New Hampshire, but looking at Iowa as a place to engage his rival.

Television ads will start later than previous years, aides said, so campaigns can save money for a long nominating fight.

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