On Main Avenue in Brookings, S.D., a smartly dressed woman spotted John Thune, the Republican running to unseat Sen. Tom Daschle, and charged across the street, her smile beaming and her hand outstretched. "I'm with you," 62-year-old Connie Burdick told Thune. "Something's got to be done," she said, to stop the downfall of traditional values. It's the gay-marriage issue that riles Burdick, an issue that Thune is betting will knock down the Democratic Senate leader.

Days before the Senate failed to advance a measure that would change the Constitution to forbid same-sex marriage--Daschle voted against it--Thune went on the attack. Barnstorming the state in a twin-prop plane, he crisscrossed South Dakota to warn that gay marriage could be looming. In a radio ad, he charged that the "institution of marriage is under attack from extremist groups," adding ominously: "They have done it in Massachusetts and they can do it here." Daschle, who has served in the Senate since 1986, opposes gay marriage, but believes that amending the Constitution is too drastic a step.

In places like Brookings, with a twirling barber pole and a grain elevator, people pride themselves on being close to the soil and to God. The tanning salon and the gym have Bible verses on the wall. This is a state that gave George W. Bush a 22-point victory four years ago, and Daschle's foes see him as out of step with the conservative values of the high plains. A clear majority of South Dakotans favor an amendment to ban gay marriage, says William Richardson, chairman of the political-science department at the University of South Dakota. "The question is, how passionate are they about this issue?"

That question is echoing across the country. The issue is likely to show up in voting booths in at least 11 states--all of them considering measures to ban same-sex marriage. Republicans are hoping it will rally social conservatives in key battleground states like Michigan and Oregon, helping the GOP ticket from top to bottom.

The South Dakota race is drawing special attention; it could help decide control of the Senate (made up of 51 Republicans, 48 Democrats and one independent heading into this fall's election). Observers expect it to be among the closest in the country. A Daschle spokesman, Dan Pfeiffer, says Thune is clearly making the gay issue "the centerpiece of his campaign," because he lacks a compelling rationale for dumping Daschle--who says his big-dog status in Washington helps keep South Dakota on the map. But Thune hopes the issue, along with possible voters' weariness with their long-serving incumbent, will carry the day.

Thune, 43, 6 feet 4 and tan, with the good looks of a television anchor, doesn't drink, smoke or swear. An evangelical Christian, he does not believe in evolution. Riding in a silver Tahoe past rolling fields choked with corn and soybeans on Interstate 229, he charged that gay marriage would mean "it's going to be taught in the schools as the moral equivalent" to the union between a man and a woman.

Still, it's unclear how well that message will draw voters to the polls. At a coffee shop on Main, Rob Rasmussen, a bearded 50-year-old, shrugged his shoulders. "I'm much more concerned about things like health insurance," said Rasmussen, who owns a bicycle shop in town. "I worry about our troops, the economy. Gay marriage--that's way down the list."

These are relatively prosperous times on both sides of the Missouri River, where the seed caps give way to cowboy hats. The value of grain has been on the upswing, and yields have been high. The price for cattle has soared. About 25 percent of South Dakotans have jobs connected to agriculture. The hottest topic in this state is ethanol, the corn-based fuel, and efforts to boost its use around the country.

In the shops and saloons of South Dakota, people are more likely to be talking about ethanol than gay marriage, at least for now. But it's still early in the political season. Come harvest time, for crops and for votes, it will become clearer what took root in the heartland.