McDonough, Ga. — In a sun-dappled square decorated with scores of entrants in the community’s Halloween scarecrow contest, a balky sound system enables, if barely, the Republican U.S. Senate candidate to exhort a few hundred people, mostly supporters, to urge neighbors to vote to reduce Sen. Harry Reid to minority leader. The exhorter is David Perdue, a glutton for punishment who has been campaigning incessantly for 15 months and may be doing so for two more.

A Jan. 6 vote would end his second runoff, which will be necessary if, because the libertarian candidate gets perhaps 5 percent of the vote, neither Perdue nor his Democratic opponent, Michelle Nunn, wins 50 percent of the vote next Tuesday. Perdue’s first runoff, a nine-week slog ending in July, resulted when none of the seven Republican primary candidates reached 50 percent.

While the grandson of former Gov. Jimmy Carter runs for governor, Perdue, the cousin of Georgia’s two-term governor Sonny Perdue, is running against the daughter of former four-term senator Sam Nunn. Barack Obama’s unpopularity is her principal problem, but bucking such headwinds is a family tradition. In 1972, when her father won his first term, George McGovern led the Democratic ticket, en route to losing 49 states, including Georgia by 50 points. Nunn won by eight.

That she might win Nov. 4, or at least force a runoff, illustrates a paradox of Republican politics: Republicans prefer the private sector to the public. Americans profess admiration for markets and those who prosper in them. But voters can recoil from market rationality.

Perdue, 64, won the nomination by stressing that he is a stranger to politics and is a practicing capitalist (“I would be the only Fortune 500 CEO in the Senate”), thereby touching two Republican erogenous zones. But capitalist rationality is more beneficent than pretty, which is a problem.

Nunn, 47, is a political novice from the nonprofit sector, which is doubly ideal: She has no record in any office to attack, and she has never made the political mistake of making a profit. In Perdue’s attempts, sometimes unsuccessful, to revive failing companies, he outsourced jobs, making him vulnerable to reprises of the attacks on Mitt Romney in 2012. Romney was first attacked as a “vulture capitalist” by rivals in the Republican primaries. Nunn is recycling attacks that Perdue’s Republican rivals began, and she is using an advertising firm that made anti-Romney ads.

It will be difficult for Nunn to reach 50 percent in this still red state that Romney won by only 7.8 points, compared to his victory margins in the contiguous states of South Carolina (10.5 points), Alabama (22.2) and Tennessee (20.4). And in a runoff, with the national excitement having subsided, turnout will be down, especially among three crucial Democratic constituencies — minorities, young people and unmarried women.

By then, and for similar reasons, a likely runoff in Louisiana on Dec. 6 may have sealed or enlarged a Republican Senate majority, reducing Georgians’ interest in sending a new senator to join the Democratic minority.

Sam Nunn was elected just 22 months after Lester Maddox, the pick-handle wielding segregationist proprietor of Atlanta’s Pickrick restaurant, ended his term as Georgia’s governor. But as has been said, the past is another country. The rally here occurred at the base of a monument common to squares in Southern county seats, a statue of a soldier facing South, high atop a column the base of which is inscribed with florid prose about the Confederate dead “who sleep beneath the sod of every Southern state” and “those who like a benediction, still limp in our midst.” The rally was watched by a few members of Georgia law enforcement, blacks and whites in a brotherhood of boredom, cloaked in crisply pressed uniforms and the dignity of the state. They dignify the state by the professionalism of their presence, which is evidence of how much America has changed. At the end of a political season of splenetic rhetoric, this tableau is now a remarkably unremarkable reminder of the good that politics can do.