Former Northern Virginia representative Herbert E. Harris II has pulled so close to Republican Rep. Stanford E. Parris that their spirited congressional race is almost a tossup, according to polls for both campaigns.

A poll conducted for Harris, the Democratic challenger, showed him leading by two points, while Parris' poll gave the incumbent a seven-point lead, down from an 11-point margin he enjoyed in February. Both surveys have margins of error of about 4 points.

The polls showed the economy to be the major concern of most voters in the district, but campaign spokesmen disagreed on which candidate will be helped by that concern. The race is being closely watched by both national parties as a measure of the vulnerability of the freshman Republican congressmen elected in the Reagan landslide of 1980.

"We interpret the poll as a dead heat," said Harris, whom Parris unseated in 1980 by a margin of 1,100 votes out of almost 200,000 cast. "I think even they now are down to recognizing a very close race."

"The Parris people have said continually this will be a very close race," said campaign spokesman Dick Leggitt. "And it is . . . . The only thing is, Herb's not moving up."

Leggitt said Parris' diminished lead stemmed from Reagan loyalists temporarily abandoning Parris for the "undecided" column after Parris voted against the Reagan-supported tax increase last month.

Spokesmen for both campaigns differed on the possible significance of third-party candidate Austin Morrill, who proved to be almost unknown to those sampled in both polls, and on the extent of disenchantment with Republican policies among the district's huge number of federal workers and federal retirees. They agreed, however, that most voters have strong opinions about Harris, 56, and Parris, 53, who have faced each other twice before, with each candidate winning once.

"We couldn't convince Herb Harris supporters to vote for Stan Parris if we stood on the corner passing out $10,000 checks," Leggitt said. "And you couldn't convince Stan Parris supporters to vote for Herb if he was running against Mickey Mouse."

Both polls were conducted in mid-September, shortly after the campaigns had begun advertising on radio. The Parris poll interviewed 400 likely voters in Alexandria and Fairfax, Prince William and Stafford counties and was conducted by Tarrance and Associates of Houston. The Harris poll telephoned 450 likely voters and was conducted without charge to Harris by the polling arm of the Democratic Study Group.

The Parris campaign questioned why Harris released the most recent poll after refusing to disclose the results of earlier surveys conducted by pollster Peter Hart.

The Harris campaign defended the accuracy of the Democratic Study Group poll and said earlier surveys had shown similar trends. Harris said the release of the September poll was "a pragmatic decision."

Harris said the campaign is all about federal workers and retirees -- between one-third and one-half of the district's electorate -- and he said his poll shows increasing disenchantment with Reagan and Parris among both groups. In head-to-head trial heats in the Harris poll, the Democrat led 44 to 39 percent, with 17 percent undecided, but among voters from households with federal workers, Harris held a wider 45-to-36-percent lead.

In the Parris poll, on the other hand, the incumbent led by 46 to 39 percent, with 2 percent for Morrill and 13 percent undecided. The Republican poll showed Parris holding about even among federal employes, and even Harris' poll said the congressman's favorable rating is about the same among federal workers as among the general electorate.

Both polls showed 46 percent of the wealthy, highly educated suburban district giving Reagan favorable marks, while 52 percent disapprove of his performance. In what could be a crucial test, Parris was backed by most Reagan supporters in his poll but was also favored by 35 percent of voters who think Reagan's performance is "only fair."