Incumbent Del. James H. Dillard was being challenged yesterday in the 19th Legislative District's Republican Party primary by a coalition of three ultra-conservative candidates.

Dillard, who had been assailed as being too liberal for the Republican Party by the conservative trio, was expected to win his party's endorsement, however, along with his two co-incumbents, Warren E. Barry and Robert E. Harris and two of the challengers: Robert L. Thoburn and Lawrence D. Pratt.

In an uneventful Democratic primary, the two incumbents, Richard L. Saslaw and Thomas J. Rothrock were expected to get their party's nomination together with two political neophytes, D. Dashiell Sibley and Vernon L. (Lee) Strang, and Gladys Keating, making her third bid for one of the five delegate seat in the 19th District.

The 19th Legislative District comprises Fairfax County between Rte. 236 and the southern county line. County Electoral Board Secretary Millard C. Rappleyea was predicting a turnout of about 25 per cent of Fairfax County's 239,313 voters.

The Republican wunners in yesterday's primary will be vying with the successful Democrats during the next five months for the 19th district's five seats in Virginia's 100-member House of Delegates. Under the state's multi-member district system, all five winners in the November general election represent the whole district.

During the Republican primary campaign, both Dillard and Barry, who already have served three and four terms as delegates respectively, admitted that they felt threatened by the conservative team - sometimes called the "Three Muskeeters" - and were putting a greater effort into their campaigns than they would otherwise have done.

A letter mailed to Republican voters last week that attacked Dillard may actually have been a blessing in disguise for the Fairfax county school teacher, according to some Republican Party workers.

Signed by Kathy Teague, a Fairfax County Republican Committee member, the letter said "Jim Dillard is much more liberal than the Republican mainstream of our area." It attacked Dillard's support of collective bargaining for county employees. While none of the three conservative candidates said they had prior knowledge of the letter, Thoburn confirmed that Teague wrote up a flyer for the three men that was prepared for distribution yesterday at the polls and John W. Adams, the third member of the trio, said he was "pleasantly surprised" to read the letter.

Both Dillard and Barry have the same position on collective bargaining: they both support legislation that would allow negotiations between government employees and local governing bodies provided there is a no-strike provision and no affiliation with a national union.

Among party workers, the Teague letter was not well received and even "deplored as not appropriate" because of the personal attack on Dillard. Some Republican workers "got their back up and worked even harder for Dillard than they might have" because of the letter, according to Barbara Hildenbrand, Republican chairman for Virginia's Eighth Congressional District.

Harris, whose views are less liberal than Dillard's and Barry's had said during the campaign that he was confident of renomination. A business executive with Rockwell International in Washington, Harris had been criticized for his long absences from the General Assembly while it was in session.

Thoburn is founder and administrator of the private Fairfax Christian School. Pratt, who works as a lobbyist for Gun Owners of America and is executive director of the American Conservative Union, received the endorsement of the National Rifle Association during his campaign. Adams, the third member of the conservative trio, is a partner with his brother in a paint and wallpaper company.

Taxes, increasing utility rates and collective bargaining for government employees were the issues in the Democratic Primary race but debate was almost nonexistent since the six contenders shared similar views on all the topics.

The two incumbents, Saslaw and Rothrock, emphasized their records while Keating, Strang and Sibley stressed their involvement in community affairs during the primary campaign.