JACKSONVILLE, Fla. — To hear Newt Gingrich tell it, the dramatic conservative change he promises will begin even before he is sworn in as president in 2013.

“My goal would be by the end of that first day, about the time that President Obama arrives back in Chicago, that we will have dismantled about 40 percent of his government,” he tells audiences.

Obama’s health care bill? Repealed. Legislation that toughened regulations on Wall Street after the economic collapse of 2008? History. White House czars? Gone.

All this, and more, he pledges to get done before his head hits the presidential pillow for the first time. It’s characteristic Gingrich — bold and rich with details that lend credibility and evoke applause from supporters, yet sometimes based on assumptions that strain the imagination.

It’s all designed to make the case to Republican primary voters that he, not rival Mitt Romney, is capable of envisioning and then ushering in a new conservative age.

“We need someone who is going to fight back and doesn’t back down,” said Harry Berntsen, a Gingrich supporter, after listening to the former House speaker on Sunday at a sun-splashed rally at The Villages, a mammoth central Florida retirement community.

As he has done elsewhere, Gingrich outlined his Day One scenario on Monday for a small audience in Jacksonville, Fla., as he embarked on a final, full day of campaigning on the eve of Florida’s Republican presidential primary.

The polls make Romney a heavy favorite in the state, dampening Gingrich’s hope that his win in South Carolina on Jan. 21 portended a steady rise. Already, Gingrich is pointing toward caucuses in Nevada and Minnesota in early February, followed by a showdown in Arizona at the end of February and Texas in the spring.

The closest approximation that Romney has to Gingrich’s opening-day narrative is a pledge to sign an executive order allowing the states to opt out of the health care law, preferring to stress his credentials as a businessman while campaigning as the man who knows best how to create jobs.

The new Congress will convene on Jan. 3, 2013, he notes, while the presidential term begins at noon on Jan. 20.

“I will ask Congress to stay in session” in the meantime, Gingrich said, so that legislation is ready for his signature on Inauguration Day that repeals the health care bill that stands as Obama’s top domestic accomplishment, wipes out the so-called “Dodd-Frank” legislation that imposed new regulations on Wall Street and scraps a 2002 measure that toughened accounting rules.