In Southside, the new congressman is a 55-year-old millionaire soft-drink bottler, son of a Jewish Lithuanian immigrant, a Democrat who unseated a 10-year Republican incumbent with a campaign that featured a heavy dose of television, the poor economic health of the region and the Democrat's own gregarious style.

In the mountains of the southwest, another Democrat, a 36-year-old state senator and one-time Wall Street lawyer, beat a popular 18-year Republican incumbent with a campaign that, like the one in Southside, stressed the failures of Reaganomics.

In the Roanoke Valley, a third Democrat won in a fight for a traditionally Republican seat. The winner, a 62-year-old retired General Electric executive who sees Congress as a second career, is an Illinois native who campaigned as a fiscal conservative in the Virginia tradition but looked and sounded more like a suburban moderate.

The fact that three of Virginia's 10 congressional seats shifted from the Republicans to the Democrats last week eased the Democrats' pain over the defeat of their candidate, Lt. Gov. Richard J. Davis, in the Senate race.

But more interesting than the new lineup in the delegation -- now six Republicans to four Democrats -- was the type of Democratic candidate who won and where they won. And where they didn't win.

Traditionally, it has been the Washington suburbs that have swung with the national trends. In 1974, the year of Watergate, the Republicans lost both Northern Virginia's 8th and the 10th congressional districts. In 1980, the year of Reagan's victory, both seats returned to the GOP.

Last week, as Republicans around the country fell victim to discontent with Reaganomics, the Republicans in the two districts stood pat -- in part, some said, because of the region's insulation from the recession. This time, however, the national trends passed over the Rappahannock River and settled in downstate Virginia where the local economy has been harder hit.

Also this year, voters in the three downstate districts were offered fresh alternatives, put forward by well-financed, professional campaigns. None of the Democratic candidates had run for Congress before; none had ever lost an election. They, therefore, were unhampered by old intraparty feuds and/or defeats of the sort that plagued Ira M. Lechner and Herbert E. Harris, the two Democrats running in Northern Virginia.

In fact, the three new Virginia Democratic congressmen--state Del. Norman Sisisky in the Southside's 4th, James Olin in the Valley's 6th, and apparent victor state Sen. Frederick C. Boucher in the mountainous 9th -- broke a mold. Not one comes out of the old Byrd school, a type now known in Congress as "Boll Weevils," like Virginia's only current Democratic congressman, W.C. (Dan) Daniel of Danville.

Although all three newly elected Democrats campaigned on the requisite platform of fiscal restraint, they took more moderate stances on social issues like the Equal Rights Amendment, abortion, tuition tax credits and the environment.

There were other reasons, too, for the Democrats' gains--reasons, some suggest, that made it only a matter of time before the Republicans' hold on the three districts was broken.

The Southside's 4th District, for instance, is heavily Democratic, with the largest black population (40 percent) of any district in Virginia. There, political analysts say, a Democratic victory had been in the offing for years.

Likewise, politicians note how the Democrats' strength in the mountainous 9th had been building in recent years with the influx of new voters during the coal boom of the 1970s. Ronald Reagan carried the 9th in 1980, but only barely--which, some say, should have been taken as early warning sign for Republican Rep. William Wampler, dean of the state's delegation.

Demographics also played a part in the Democrats' capture of retiring Republican Rep. M. Caldwell Butler's seat in the 6th District. The city of Roanoke has voted Democratic in greater numbers in recent year and, this year, the Democrat Olin was from the populous Roanoke Valley. His Republican opponent came from a region to the north that had been recently added to the district.

Others said the 1981 victory of Democrat Charles S. Robb provided the groundwork for the congressional victories. Robb's gubernatorial election showed that the GOP was not invincible and a moderate Democrat could appeal to a broad cross section of Virginians. And with Robb's help, Democrats served notice on the GOP that Virginia now must be considered a two-party state