During John F. Herrity's nine-month run for Congress, a run that ended Tuesday night when he failed to unseat 8th District incumbent Rep. Herbert E. Harris II, the Democrats on the Fairfax Board were happy.

Herrity, the Republican board chairman, didn't have a great deal of time to make the Democrats angry. "The board was rather pleased that he wasn't paying much attention," said Martha V. Pennino (D-Centreville), vice chairman and leader of the six Democrats on the nine-members board.

But when Herrity returns to the board's regular Monday meetings without the yoke of a congressional campaign, Pennino said yesterday, sparks may fly.

"If he tries to tighten up on the board as he did when he was elected chairman [in 1975] the board might have to get tough," said Pennino, who is considering running for Herrity's job next year.

One liberal board member, Alan H. Magazine (D-Mason), said yesterday that there will be conflicts in the coming months because Herrity is a "tiger and a tiger can't change his stripes."

For his part, Herrity, who appeared to have recovered quickly from his bitter campaign against Harris, said yesterday, "I've been through some rough situations and I think I can handle myself with her [Pennino] or anybody else."

It was Herrity's aggressive, sometimes negative campaign against Harris that may have cost him Tuesday's election, according to many Northern Virginia politicians, both Democrats and Republicans.

State Sen. Wiley Mitchell (R-Alexandria) said yesterday that Herity made a mistake in mailing a campaign letter last week that accused Harris of voting in Congress against stiff penalties for child pornographers and for federal spending on homosexuals.

"That kind of tactic just doesn't appeal to the people of Northern Virginia," Mitchell said.

Herrity said yesterday the letter, which was mailed to 3,000 voters and which aroused Harris' ire, "probably turned some people off and maybe turned some others on."

Either way, Herrity claimed that Harris' voting record on child pornography was not "some last minute thing that we pulled out of the boogeyman closet."

But Harris, reacting last week, called the letter a distortion, a lie and the worst example of mudslinging he had seen.

Supervisor Joseph Alexander (D-Lee) said yesterday the disputed campaign letter was "the crowning blow in the kind of negative campaign that Herrity has run all the way through."

Herrity's approach throughout the campaign was to attack Harris as "the biggest spending congressman in Virginia." Harris countered in their 31 joint appearances and debates that he was an "effective" congressman.

Republican Mitchell said Harris' work against the commuter tax, for Metro funding and for the Fairfax County tap on the Potomac River and his fight against Carter's civil service reform endeared him to both Republican and Democratic constituents. "The voters felt obligated to support Harris," Mitchell said.

In the neighboring 10th District, where the two-term Democratic incumbent Joseph L. Fisher staved off the challenge of high-spending Republican Frank Wolf, effectiveness was also seen as the deciding issue.

People weren't mad at Fisher," said Vincent M. Callahan, who lost to Fisher in 1976.

Fisher won with 54 percent of the vote to Wolf's 47 percent. The vote, according to Fisher's administrative aide John Milliken, broke down across Arlington and Fairfax counties as it had in Fisher's past two victories. Fisher won big in Arlington (with 58 percent) and more narrowly in Fairfax (with 51 percent). The two split a small turnout in Loudoun County.

While voters may not have been angry with Fisher, Fisher and some of his campaign workers were angry with Wolf and his frequent personal attacks on Fisher's integrity.

At Tuesday night's victory celebration for Fisher, senior campaign advisers said they were worried that "the troops (would) stay cool" when Wolf showed up to congratulate Fisher.

Wolf, who outspent Fisher nearly 2 to 1, "would be the obvious choice in 1980" to challenge Fisher again, according to Republican Callahan.

Fisher, however, will be a tougher opponent because of his senior position on the House Ways and Means Committee.

While voter turnout in the 10th District (near 54 percent) was as expected, turnout in the 8th District was somewhat lower than had been predicted.

In the southern half of Fairfax County, where election officials had predicted a 60 percent turnout, 53 percent of those eligible voted. Herrity picked up just 52 percent of the vote in the county where he had been anticipating 55 percent to offset Harris' strong showing (58 percent) in Alexandria.