Montgomery County School Board member Marian Greenblatt, a GOP candidate for the 8th District congressional seat, would like to think that on economic issues, she would represent the mainstream county voter better than incumbent Michael Barnes.

Greenblatt also would like to shift the focus of the debate in the campaign to economics, after her opening-day venture into foreign affairs produced an unexpectedly harsh backlash.

So Greenblatt has latched onto one of the most popular economic issues of the day: the proposal for a constitutional amendment to require a balanced federal budget. She is a member of President Reagan's task force studying the balanced budget issue, and she has accused Barnes of being "a part of the problem" of unbalanced federal budgets with whopping deficits.

"Mr. Barnes' answer to deficits is more taxes, more of the same tired economic philosophy . . . ," Greenblatt said last month, announcing her support for the balanced budget.

"I challenge him to denounce his bankrupt theory of tax-and-spend economics, for the good of Montgomery County taxpayers and the nation," Greenblatt said. She added that she has had experience balancing budgets: her own at home and the school board's $350 million budget.

Barnes, for his part, has adopted the position that balanced budgets are fine--he has always supported balancing the budget, says an aide--but that the proposed amendment to the Constitution is "unworkable."

Greenblatt's best-known primary election opponent, former school board member Elizabeth Spencer, said she would support sending the balanced budget amendment to the states for ratification, although she still has problems with the amendment itself.

"I'm becoming convinced that the amendment will give support to those members of Congress who support a balanced budget but have problems with their own constituencies," Spencer said.

To Republicans, many of whom had been distressed about the course of Greenblatt's campaign in the opening months, that kind of exchange brought a welcome sigh of relief. Those GOP stalwarts who insist Barnes is vulnerable think the only formidable challenge can be launched over economic issues.

Shifting the debate to economics and the balanced budget amendment also allows Greenblatt to test her theory that, on fiscal matters, Montgomery County residents are growing increasingly conservative. She has called Barnes "out of the mainstream" of county voters on economic issues, but that charge was lost amid the public clamor over her more widely publicized accusation that Barnes supports the Palestine Liberation Organization.

It was Barnes who in 1978 claimed that he represented the mainstream on the economy, in a similar lively exchange with the incumbent he defeated, Newton Steers. Steers supported the Kemp-Roth tax cut bill, which has since become the bible of Ronald Reagan's supply-side crusade. Barnes opposed Kemp-Roth, and his early stated opposition won him the endorsement of The Washington Post editorial page. On the night of his first election celebration, Barnes attributed his victory partly to his opposition to Kemp-Roth and the editorial boost from The Post.