Just over a year ago, Republican Ronald Reagan swamped Democratic incumbent Jimmy Carter 1,253 to 536 in the Stratford subdivision near Mount Vernon. Twelve days ago, in voting for governor of Virginia at that same polling place, Democrat Charles S. Robb beat Republican J. Marshall Coleman 817 to 786.

Farther up the Potomac, in the GOP stronghold of McLean, where Reagan had trounced Carter 608 to 298 in voting at the Lewinsville Elementary School, Robb beat Coleman 436 to 395.

At the same time that Robb was edging Coleman in dozens of Republican precincts in Fairfax County where Reagan beat Carter better than 2-to-l, Robb also was piling up huge margins in traditionally Democratic precincts in Alexandria and Arlington.

At Jefferson-Houston Elementary School in a largely black neighborhood in Alexandria, for example, the Robb ticket attracted 528 voters to the polls, an increase of 27 percent over the turnout three years ago when Republican John W. Warner edged Democrat Andrew Miller for the U.S. Senate. That year, only 415 persons voted at the school.

Paul Goldman, a Democrat strategist, studied the returns and tried to pinpoint where Robb accumulated his 106,000-vote victory, using the results of the near toss-up 1978 Warner-Miller Senate race as the basis of finding the difference.

Goldman concluded that the additional Democratic votes were provided by 30,000 blacks; 25,000 suburban whites; 15,000 city whites; 10,000 coal miners and their families in Southwest Virginia; 10,000 blue-collar workers around the state and 10,000 scattered elderly, federal workers, tenants and white liberals.

The 8th and 10th congressional districts provided about 21,000, or one-fifth, of the margin that helped Robb become the first governor from Northern Virginia since Westmoreland Davis of Loudoun County was elected in 1917.

Robb got about 60 percent of the vote in Arlington County and Alexnadria, where Democrats traditioanlly do well (Carter nearly carried Alexandria), but his most significant showing was in Fairfax, where he captured 51.7 percent of the vote from residents who gave Carter only 34.8 percent a year ago.

In each of the eight supervisor districts of Fairfax County, Robb managed to carry at least one precinct that went for Reagan by better than a 2-to-1 margin.

The biggest Fairfax turnarounds were in Springfield, where Robb carried Burke, Chantilly, Crestwood, Greenbriar West, Keene Mill, London, Orange and Valley; and in the Annadale precincts of Chapel, Kings Park, Lake Braddock, Laurel and Poe, all of which had given whopping margins to Reagan.

The large turnout was the key in many black and working class precincts. In Alexandria, the percentage of registered voters who showed up this year, compared to three years ago, was up 14 percent at Cora Kelly School; 12 percent at Houston School; and 7 percent at Engine Company No. 2.

Ira Lechner, a liberal Arlington Democrat, believes the last-minute advertising blitz by Reagan in Coleman's behalf failed. "It solidified wavering Democrats and independents, by helping them see a clear difference between Robb and Coleman, without helping Republicans, who knew ahead of time what Reagan would say."

"Robb ran a brilliant campaign," said Lechner, broadening the Democratic base of support to attract independents and moderate conservatives, while retaining liberals and blacks. "Robb threaded the needle."

In the Tidewater area, the turnout in many black precincts was even greater than in last year's presidential election. In Lee Precinct in Newport News, Robb beat Coleman 240 to 1. "That's phenomenal," said Goldman, who has been charting Virginia elections since the mid-1970s. "You hear about those kinds of turnouts, but you seldom see them documented."

In the 19 predominantly black precincts in Richmond, the vote for Robb was 97 percent. In the key black polling places in Newport News, the vote for Robb was 98.2 percent; 96.2 percent in Norfolk, 96.3 in Portsmouth, 96 in Paul's Branch Precinct in Suffolk, where liberal Henry E. Howell managed to attract only 89 percent of the vote.

In Southside, the heart of the plantation Virginia, the white turnout for Coleman was more than offset, said Goldman, by conservatives who switched to Robb at the urging of Virginians for Robb, the 1981 remnant of the old Byrd machine.

At state Republican headquarters in Richmond, press spokesman Neal Cotiaux said he basically concurred with Goldman's conclusions. He said he also agreed that the Reagan impact was negligible in traditional Republican strongholds, and actually worked against Coleman in some other areas.

"A 97 percent turnout in black areas was something we didn't count on," Cotiaux said. He attributed the outpouring to the final weekend television blitz linking the President with Coleman. "It didn't convince anyone to come out for us who wasn't already on our side, but it brought out a lot of people who were unhappy with Reagan," Cotiaux said.

"The biggest shocker," Cotiaux said, came from the 7th Congressional District, which stretches along the Shennandoah Valley. Although the 7th was the only one of the 10 congressional districts Coleman carried, the margin there, about 3,000 votes, was "thousands short of what we anticpated," Cotiaux said. He noted that Coleman carried his hometown of Staunton by only 100 votes.

Another major surprise to the GOP came in the 3rd Congressional District, which includes Richmond and the surrounding suburban Henrico and Chesterfield counties. "We didn't count on that," Cotiaux said, adding that the heavy black turnout in the capital city was only part of the reason for the Robb margin there. "He eroded a lot of the traditional conservative, suburban vote in the third," Cotiaux said.

But those are all impressions, Cotiaux said. He said no one at GOP headquarters has bothered to examine the results on a precinct-by-precinct basis. "We're still second-guessing what went wrong with the strategy," Cotiaux said.