Supporters of the Equal Rights Amendment (ERA) in Virginia have been waging a behind-the-scenes campaign - including telephone and door-to-door canvassing - to defeat Del. James M. Thomson (D-Alexandria), who is considered by women's groups to be the leading obstacle to ratification of the amendment in the state.

But some Alexandria Democrats have become concerned that the campaign may backfire and lead to the defeat of Richard R.G. Hobson, a pro-ERA Democrat seeking his second term in Richmond.

Alexandria's two seats in the House of Delegates are currently held by Democrats Hobson and Thomson. They are being challenged by Republican Gary Myers, an Old Town attorney who ss a strong supporter of ERA.

Although both Hobson and Thomson have insisted that they are supporting each other against Myers despite their differences on the ERA issue, a group known as Virginians for the Equal Rights Amendment (VERA) has sent literature linking Myers and Hobson and urging voters to support them against Thomson.

This upset Barbara Joseph, Hobson's campaign manager, so much that she complained to the VERA group about the literature. Since then, fliers urging support of Hobson and Myers have included a disclaimer stating that the two men are not supporting each other.

Thirty-five states have ratified ERA three short of the necessary number for it to be added to the Constitution. The deadline for ratification is March, 1979.

"To me Virginai is crucial because its one of the states that's considered impossible," said one member of the Alexanbria chapter of the National Organization for Women. "If we can do it [ratify the ERA] anybody can do it."

ABout 3,000 cheering, flag-waving ERA supporters gathered in Old Town Alexandria yesterday to hear comedienne Lily Tomlin say she had come from California "to help defeat the strongest anti-ERA candidate in Virginia and probably in the whole country." She did not mention Thomson by name, but left no doubt that she was referring to him.

Tomlin said that when Alexandrians go to the polls on Tuesday they will be voting for women across the country and she urged her audience to "put women where they've never ben before - in the Constitution."

Myers, the Republican challenger who supports ERA, told a reporter at the rally that he had been campaigning all day. "The word is out. Decisions have been made. ERA is the big issue," added Myers.

One city political observer, who asked not to be identified, sai the Hobson people are worried that the votes of a combination of pro-ERA Republicans and anti-ERA is the big issue," added Myers.

One city political observer, who asked not to be identified, said the Hobson people are worried that the votes of a combination of pro-ERA Republicans and anti-ERA Democrats and conservatives coudl lead to Hobson's defeat and the election of Thomson and Myers.

But Hobson said that yesterday's rally would help him and added that the ERA was the most frequent issue raised by the people he has talked with.

Because of tears about the campaign's backfiring and because they feel publicity about their campaign may turn out the anti-ERA vote. VERA leaders adamantly refsued to talk about it and in one case refused to even acknowledge its existence. On woman involved in the campaign offered exclusively to treveal the strategy to a reporter after the election if nothing was written before it.

"One does not lay one's strategy this early in the game," said another VERA leader.

Thomson believes the pro-ERA forces prefer to carry on a quiet campaign because "the more you explain what the ERA means the more you're in trouible in getting it approved." The ERA would ban discrimination on the basis of sex.

"It's taken a different focus in different areas of the city but its an extensive citywide campaign to defeat me on the ERA issue," Thomson siad of the VERA effort.

Thomson contends that if the ERA becomes law women will be drafted into the armed forces on an equal basis with men and that laws now on the books which protect women will be wiped out.

The 22-year veteran of the House of Delegates, who has been nicknamed "landslide Thomson" because of his many close re-election races over the years, says the courts will not be able to act when faced with a consitutional amendment that is "flat against any discrimination."

Thomson says he has been a leader in the field of equal rights for women pointing to his co-sponsorship of the ERA in the Virginia Constitution and his sponsorship of the state fair credit act for women.However, as chairman of the Virginia House Privileges and Elections Committee, he has led the anti-federal ERA forces in the House.

The VERA campaign has included a survey to pinpoint households who support the ERA. This has been followed by personal visits by VERA workers to the households, pointing out the differences among the three candidates on the ERA issue. Several pieces of Literature urging voters to support Myers and Hobson have also been mailed out.