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They might not have known it, but voters in Mansfield and Foxboro were heard from coast to coast when they cast ballots in the presidential primary on Tuesday.

That's because Mansfield and Foxboro are "bellwether towns" for presidential primaries - towns where the local election result is always nearly identical to the outcome for the state as a whole.

"Bellwethers are the communities that have the genetic code of a state, or the country," said David Paleologos, a professor at Boston's Suffolk University Political Research Center who has developed a method of projecting election results by surveying voters at polling places in bellwether towns.

Once again on Tuesday night, Mansfield and Foxboro correctly predicted the statewide result.

"It's just mind-boggling," Paleologos said. "It's amazing to see the consistency in the numbers." In national politics, the classic bellwether state is Missouri, which has supported the winning candidate in every presidential race but one since 1904. Once upon a time, Maine was also seen as a bellwether; hence the old saying, "As Maine goes, so goes the nation."

Only six of Massachusetts' 313 municipalities are bellwether towns for presidential primaries, Paleologos said. The other four, aside from Mansfield and Foxboro, are Millis, Nahant, Stoneham, and Waltham.

Paleologos also serves as chief pollster for WHDH-TV (Channel 7), the NBC affiliate in Boston, and in that role he helped NBC News become the first media outlet to call the Bay State's Democratic primary in favor of the eventual winner, Hillary Clinton, on Tuesday night.

Earlier that day, the exit polls used by most of the press had showed Clinton's chief rival, Barack Obama, with a big lead in the Bay State, leading many to expect an Obama victory.

But when Paleologos saw the results from Mansfield and Foxboro, along with the other bellwether towns, he correctly projected Clinton would be the eventual winner. NBC was the first to broadcast the news, leaving the other outlets scurrying to catch up.

"I just thank the people of Mansfield and Foxboro for taking part," he said.

Paleologos is excited about the possibility of using bellwether towns to predict election outcomes more accurately in the future. The model has not been used in the past, he said, because it's so time-consuming to aggregate election returns from so many places required to find the bellwethers.

To find a bellwether town, Paleologos and his assistants analyze the election returns for every city and town in a state, and then find which ones match the statewide results, plus or minus a few percentage points, in two consecutive election cycles.

The designation of a bellwether town is extremely case-specific. For instance, Paleologos' research has found that Mansfield and Foxboro are bellwether towns only for presidential primary elections where both parties have open contests because there is no incumbent president. Before this year, the most recent examples were 2000 and 1988.

On the other hand, Mansfield and Foxboro are not bellwethers in gubernatorial primary elections. In 2006, Paleologos used other towns to predict those results. "And if we assume for the sake of argument that whoever gets elected president gets re-elected," he added, "then (Mansfield and Foxboro) probably won't be a bellwether again until the presidential primary in 2016."

As for the reason Mansfield and Foxboro happen to be bellwethers, Paleologos' theory is that the profile of independent voters in each town closely tracks the profile of independent voters across the state.

Although both towns have grown since 1988, the population changes apparently have not been enough to make Mansfield and Foxboro unrepresentative.

In the years to come, Paleologos hopes that bellwether towns can help improve the accuracy of political polling.

"We don't have a crystal ball, but it's like we're a doctor that does three tests and another doctor does one - we've got more information," he said. "It makes us more confident."