Far from Detroit, across an emerald ocean of flourishing corn and soybeans, the burning questions here -- and believe it, the argument is heated -- is whether Ronald Reagan was born on the east side or the west side of Main Street.

The answer is important, one, for reasons of historical accuracy, and two, because if this northern Illinois farm hamet becomes the 1980s Plains of the plains, as it were, someone will turn a nice dollar.

Reagan was born here in 1911 and left for good before he was in his teens. But even today the place seems entirely plausible as a fount of the conservatism that Reagan has made his stock in trade.

The population is about 850. Tampico's resident's, for the most part, are Republicans by birth and by belief. The surrounding farms are immaculately kept and the land is enormously fertile. Tampico is dominated by a chalk-white grain elevator that towers near the single strand of railroard tracks.

Tampico and the rest of Whiteside County are rock-hard conservative even through, ironically, Reagan's late father, a poor itinerant merchant, was a liberal Democrat who led his son to think the same way for a while.

One of Reagan's childhood playmates, now one of the area's few Democrats, is a farmer named Albert (Ab) Forward. He calls Tampico so conservative that "if the Democrats had Christ Himself on the presidential ticket, he couldn't get elected here."

Forward, it turns out, is one of the figures in the dispute over the site of Reagan's birth that has blocked just about everything else out of local conservation. "I've caused a lot of commotion around town," Forward acknowledged.

The dispute reached a new pitch last weekend when the Sterling Gazette printed an article quoting Forward and challenging the old assumptions about Reagan's birthplace.

Forward contends that Reagan was born on the west side of Main Street, in a second-story flat above the general store where Jack Reagan sold shoes and other goods. Today, the building houses the Tampico Lounge -- a combination bar, pool hall and lunchroom. A sticker on the beer cooler says, "Avoid hangovers: stay drunk."

But the weight of history seems to rest more comfortably across Main Street, in the old First National Bank Building, which many Tampico residents want to see made into an honorary national headquarters for the Reagan campaign.

The building is owned by Paul and Helen Nicely, who have compiled an array of documentation to back up their belief that Reagan was born there when the first floor was occupied by a restaurant.

The Nicelys would like to see the building restored and preserved as a historical site or research center if Reagan is elected. But they're a little exasperated by the controversy.

"It's the tavern crowd stirring this up," Nicely said. "It has always been assumed Reagan was born here and he was in this building four years ago. Nobody challenged the assumption until now."

County GOP precinct committeeman Vernon Dennison, 68, another Reagan boyhood playmate, agreed with Nicely. "Well, anyway, he was born here and they can't take that from Tampico."

Dennison and others already are speculating about what will happen to Tampico if Reagan wins in November. The tiny downtown is pocked with sites that could spawn tourist shops. And there still are people here who claim to remember the Reagan's some rather vividly.

One of them is Gladys Pierce, 73, who runs a cleaning shop and collection office for the utilities. She has a Reagan portrait in the front window, some GOP bumper stickers and inside, next to a Reagan post card, a picture of President and Mrs. Jimmy Carter.

Other memorabilia adorn the walls, including pictures of John Shaheen, the famous oil man, and his sister Ruth, who became Mrs. Hugh Downs. Both are from Tampico.

"Ronald was just a typical boy. He used to come to our farm to ride a pony with my brother Gordon, who was his classmate. Before Gordon died, he told me Ronald would never make president because he's too old," Pierce said.

As Pierce talked and brought out stacks of Reagan snapshots, a radio behind here blared news from the GOP convention in Detroit, news of public opinion polls suggesting that Gordon may have missed the mark on that prediction.

Thirty miles to the northeast, in Dixon, another of the peripatetic Reagan family's stopping places, there is no controversy. The city of 18,100 is, rather, basking in the glow of its new recognition.

Reagan spent his teen-age years in Dixon, which now happens to be represented in Congress by one Rep. John B. Anderson who also happens to be running for president, as an independent.

The rush of the national news media to get background on the GOP candidate has set off a small boom in Reaganiana.

Journalists are led to interview retired teacher B. J. Fraser, Reagan's drama coach at Dixon High School, and Helen Lawton, a friend who has Reagan's portrat taped to her front door. Other high points on the tour are the Rock River Beach, where Reagan was a lifeguard, and a bridge which ws dedicated to Reagan in 1978.

Old-timers may remember Reagan and tell and retell their stories, but for today's schoolboy its quite another thing, this Reagan candidacy.

When the Dixon Petunia Festival this summer sponsored an essay contest for grade-schoolers, challenging them to name the person they most admired in Dixon history, only two of 108 picked Ronald Reagan.