With the surprise defeat of their two prized congressmen, Northern Virginia Democrats may want to take a close look at their pockets of voting strength when they take up the delicate subject of redistricting next year.

Specific plans for carving up the region's political jurisdictions do not exist yet. But it's a safe bet that when all the census data is finally in, the pressure will be enormous to group Democratic strongholds together and, thus, ensure at least one Democratic seat in Congress for Northern Virginia.

After reviewing the results of last Tuesday's election, for instance, some Democrats say they would not be averse to putting Arlington and Alexandria in the same district. Both jurisdictions, Alexandria in the 8th congressional district and Arlington in the 10th, had a majority of voters who stuck with the Democratic incumbents for Congress, even though they supported Ronald Reagan for president.

"It's an obvious idea, and it would have been talked about regardless of the election outcome," says Del. Elise Heinz, who represents Arlington and Alexandria in the General Assembly.

The two political subdivisions were once a part of the same congressional district, Heinz adds, and the idea of reuniting them ought to be examined.

The defeated incumbents, representative Herbert E. Harris of the 8th District and Joseph L. Fisher of the 10th, have long opposed any reapportionment that might cost them large blocks of voters. But with Harris and Fisher out of the picture -- and replaced by Republicans Stanford E. Parris and Frank R. Wolf, respectively -- those objections hold little credence.

What does hold credence, particularly for the anxious-to-make-a-comback Democratic Party, is "at least 12 young Democrats salivating" to run for the 8th and 10th district seats in 1982, according to one party lawmaker.

The people who will be calling the shots in any reapportionment plan are members of the Democratic-controlled state legislature, and that should make any party favoritism easier.

Grouping Arlington and Alexandria into the same congressional district "is exactly what I'd do if I was a Democrat in the legislature," affirmed one Parris aide. "You could draw a district that no Republican could ever win."

The aide, Dick Leggitt, said the GOP would be actively involved in the reapportionment process to ensure that district lines are drawn fairly. But he added that the Parris camp is not overly concerned because "my boss lives in three places in the area, and it would be very difficult for them to gerrymander us."

Political considerations aside, "Alexandria and Arlington share a lot in common that we don't share with the outer suburbs," argues Del. Warren G. Stambaugh, an Arlington Democrat who supports the merger idea. "They're both the urban care of the region, and the area inside the Beltway would just be a more natural congressional district than combining us with Loudoun or Prince William County."

Stambaugh is clearly aware of the political benefits of such a merger. "There's a tendency in both communities to support good Democrats," he said, "so it would be politically helpful to have both of them in the same district."

State Sen. Adelard L. Brault, the area's senior legislator and an influential member of the committee that will draw up reapportionment plan, says the idea of combining Arlington and Alexandria "is certainly a possibility." He cautions, however, that the subcommittee set to study the whole issue hasn't even met yet.

Area Republicans, understandably, are not rushing to help carve up a new district that would be a Democratic power base. Even some Democrats argue that there is no "practical way" to make any radical shifts in existing political divisions.

One Republican, state Sen. Wiley F. Mitchell Jr. of Alexandria, already is punching holes in the merger theory. Arlington and Alexandria together, he notes, would make up less than half the 500,000 to 520,000 people each congressional district must have under new redistricting plans. Any revised district would still have to borrow population from Fairfax County or the more rural outer counties, he says.

Del. Vincent F. Callahan (R-Fairfax) dismissed the idea of a merger as "too obvious" an attempt at gerrymandering. The plan would invite a veto by Republican Gov. John N. Dalton, though Callahan admits there is similar talk of "chopping up the Tidewater area to help the Democrats."

And Representative-elect Wolf, of the 10th District, predicted any political maneuvering would cause resentment among voters.

Whatever the final makeup of the new districts, area Democrats and Republicans seem to agree that neither party can every again think of the 8th and 10th congressional seats as safe.

"How the ball bounces now is anybody's game," said Alexandria's Mitchell.