CHIPLEY, Fla. -- On a clear October morning, Florida’s senior senator stood on the red clay soil near his grandfather’s grave and pointed to the cow pasture behind him.

“I remember my bare feet on that cold earth that had been turned up by the big plow,’’ he told friends and relatives at the church cemetery halfway between Pensacola and Tallahassee. “These are the pioneers that saw technology change our way of life.”

Four hours later, Nelson was in Tallahassee, pointing again — this time at the world’s largest magnet housed at the National Magnetic Field Laboratory at Florida State University.

“We are going to Mars,’’ he told the scientists. “We need to create a magnetic field around our astronauts so if there is a solar explosion, they won’t get fried. Can you do that?”

“Yes,’’ answered Greg Boebinger, the lab director. “It’s conceivable.”

It wasn’t much of a campaign day for Nelson in this low-key re-election campaign, but it was a lot like his political career: book-ended by a pilgrimage to his roots and an homage to Florida’s technological future.

After nearly 40 years in public office, Nelson has bridged the generations and the technological divide. He has watched its cow pastures transformed in the wake of the state’s population boom. He was a civilian crew member of the 1986 space shuttle Columbia and is now the lone Democrat to hold statewide office in the nation’s largest swing state. His centrist positions on fiscal and social issues, and his low-key demeanor have helped him remain in office even as political power in Florida has shifted from Democrat to Republican. He is arguably the last of Florida’s old-style Southern Democrats.

But if Republicans have their way, the state’s longest-serving Democrat will be ousted this year.

Nelson has maintained a steady lead in the polls over his less experienced challenger, Republican Connie Mack IV. But a barrage of attack ads financed by Super-Pacs run by the U.S. Chamber of Commerce, Karl Rove, and the Koch Brothers-financed Americans For Prosperity, have taken their toll. Real Clear Politics’ average of recent polls has the margin just over five percentage points in Nelson’s favor. A new Tampa Bay Times/Bay News 9 poll of the bellwether I-4 corridor has Nelson leading Mack there by a narrow three points, 47 percent to 44 percent.

Nelson, a Yale-educated lawyer, has made being a moderate in a polarized political world the theme of his campaign for a third six-year term. But Mack, a Cape Coral congressman, portrays Nelson as a “lockstep liberal” with a penchant for big government and a disdain for freedom.

“You say one thing to the people of Florida and you do something else in Washington, D.C.,’’ Mack told Nelson during their sole debate. His arguments often distill Nelson’s positions down to distortions of his votes on taxes, health care and support of the military.

“Extremist,’’ Nelson says of Mack’s charges, his slow, southern drawl emphasizing the “t.”

That’s about as nasty as Nelson gets.

“You won’t find Bill Nelson on the extremes or making outrageous statements,’’ said Bruce Smathers, Nelson’s college roommate, long-time friend and a former Florida secretary of state. “His upbringing and his attitude about politics is that the only way to get things done — to enact things that will remain enacted — is to have both parties come together.”