CHIPLEY — On a clear October morning, Florida's senior U.S. 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, Bill Nelson was in Tallahassee, pointing again — this time at the world's strongest 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 bid, 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, 70, 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 have taken their toll. RealClearPolitics' average of recent polls has the margin just over 5 percentage points in Nelson's favor. A new Tampa Bay Times/Bay News 9 poll of the bellwether Interstate 4 corridor has Nelson leading Mack by a narrow 3 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 Fort Myers area 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, longtime 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."Nelson laments that Mack has embraced the petty partisanship that has "polarized Washington." It's an approach, Nelson says, that is unlike his father, former Sen. Connie Mack III, whom Nelson replaced in the U.S. Senate.But Nelson has stayed clear of frequent references to President Barack Obama. He appeared at a campaign stop with Vice President Joe Biden, whom he considers "a close friend," but rarely mentions the president's name.He likes to say that when he flew in space, "I looked back at Earth and I didn't see political division. I didn't see religious division and I didn't see ethnic division.'' It is a recitation of his latest television ad, and the closing message of his campaign."It's the message that everybody wants,'' Nelson said last week. "It's the message that my 104-year-old grandmother said on her death bed: 'Son, don't think of yourself as better than other people.' " Nelson's highest profile accomplishments in Congress have not only straddled the center, they have focused on Florida.When funding was drying up for the space shuttle program, Nelson worked with Republican Sen. Kay Bailey Hutchison of Texas to sponsor successful legislation to ensure that NASA would remain committed to a space exploration program. When pressure was mounting on the George W. Bush administration to open the Gulf of Mexico to expanded oil and gas exploration, Nelson teamed up with then-Sen. Mel Martinez, a Republican, to author a bill to keep Florida's shores safe from oil drilling.When it appeared that BP's fine dollars were being diverted from gulf businesses, he worked with Sens. Mary Landrieu, a Louisiana Democrat, and Richard Shelby, an Alabama Republican, to pass the Restore Act this summer. The measure dedicates 80 percent of the fines from the Deepwater Horizon oil spill to restoring the Gulf Coast's environment and economy.He has championed a host of populist — albeit safe — causes: sounding alarms over Burmese pythons in the Everglades, decrying the import of defective Chinese drywall, banning arsenic-treated wood, demanding pension for former Negro League ballplayers in Tampa.On social issues, Nelson also plays it down the middle. He personally opposes same-sex marriage, for example, but believes it should be a decision left to the states.He voted in favor of the Affordable Care Act, but insisted on passage of a compromise to mitigate the impact of cuts to the Medicare Advantage program, the private insurance option that covers nearly 1 million Florida seniors. Nelson is rated as a centrist by the government watchdog GovTrack.us, which relies on votes and bill sponsorships to analyze the records of congressmen. Mack, by contrast, is listed as a "rank-and-file Republican." Nelson counts among his closest friends several senators who take part in a weekly prayer group in Washington, including Republican Sen. Orrin Hatch of Utah; Sen. Joe Lieberman, an independent from Connecticut; and Republican Sen. Lamar Alexander of Tennessee. He also notes that Florida's junior U.S. senator, Marco Rubio, "has not come out in a position and said anything against me in this campaign."Nelson's fence-sitting record has also come with a price. His popularity is lower than average for a veteran incumbent, said Brad Coker, director of Mason-Dixon polling, which conducted the latest poll for the Tampa Bay Times and Bay News 9. "Bill Nelson has been in public office a long time and to have favorability numbers of only 39 percent is not terribly impressive,'' Coker said. "It's a sign people are not thrilled with him."Nelson attributes the voter distress to the size and diversity of the state, as well as the onslaught of negative attack ads mounted against him by out-of-state groups — $21 million by his count. Nelson has raised $16 million and, he tells audiences, "I've had to do it on my own."Nelson is not ready to say that, if re-elected, this will be his last campaign. "Do I want to slow down? No,'' he said firmly, noting that he doesn't drink, still jogs, and works out daily. He examined the assembled group of aides, students and reporters on the tour at Florida State and concluded: "I bet you, looking around at this crowd, I can do more push-ups than anybody here." Mary Ellen Klas can be reached at [email protected]iamiherald.com. Follow her on Twitter @MaryEllenKlas.