Democratic Rep. Abner Mikva, whose possible defeat represents a last symbolic Republican hope in a disappointing national campaign, abruptly changed the subject from economics to gun control a week before the election.

Addressing students at Evanton Township High School, Mikva pulled from his pocket a National Rifle Association (NRA) letter attacking him as "the foremost anti-gun spokesman in the House" and calling Republican John Porter "immensely preferable." In Chicago's North Shore suburban district, a candidate with the NRA as a friend needs no enemies. Asked by newsmen about it later that day, state Rep. Porter protested that his position on handguns is identical to Mikva's.

To political insiders, Milva's sudden recourse to gun control was a distress signal. In rising from a 16-percentage-point deficit to a virtual dead heat, Porter has exploited his own support for tax reduction and Mikva's liberal spending record. Reversing the traditional pattern, pocketbook issues were running so decidedly in the Republican's favor that the Democrat felt compelled to switch to social issues.

This lavishly financed race represents what Republican strategists had hoped would be the national pattern. But the single act of defeating Mikva, the most effective, influencial liberal in the House, would partly compensate for dreary GOP showing elsewhere. "Beating Ab Mikva would give us at least something this year," a national party leader told us.

Accordingly, organized labor and big business have opened their war chests for Mikva and Porter. Jimmy Carter and Jack Kemp scheduled late visits especially for this contest (following Walter Mondale, John Rhodes, Gerald Ford and Edward M. Kennedy). Henry Fonda left Broadway to tape a Mikva television spot. No other House race gets such attention.

The reason is Mikva. Of 1977 test votes selected by the Americans for Democratic Action on which Mikva actually voted, he was 17 for 17 on the liberal side. Nobody else with that record could dream of surviving in this swing district. The perception of four-term Mikva as an exceptional man of character carried him to narrow wins in 1974 and 1976 (after losing in 1972) against Rep. Samuel Young and nurtured hopes of an easier win against the less well-known Porter.

But Mikva this year pooh-poohed the force of the tax revolt. On the House Ways and Means Committee, he dug in against tax relief for the middle-to-upper income brackets. In a district where nearly half the voters earn over $25,000, that is high-risk politics.

What's more, Porter has avoided corrosive social issues that attract Republican candidates like the candle beckons the moth. He endorses the "strong social conscience" of this district and sees little differences between him and Mikva on social questions (which is fiercely denied by Mikva). Rather, Porter declares that "economic upward mobility has come to a halt in America" and demands tax cuts and reduced government to change that.

Porter for Congress, for all of us" is given anti-Semitic connotations by some Mikva backers. Actually, Porter wisely dropped the old theme that this formerly WASP preserve shound not be represented by Abner Mikva. Instead, his newspaper and radio ads relentlessly sound the theme that Mikva has consistently voted for massive spending - except for national defense.

That charge led to a nasty private exchange between these two Evanston neighbors after their last debate. In truth, Mikva voted against the last three House defense appropriations bills (while supporting the final conference committee version.) More signigicant, he voted against the nuclear aircraft carrier and B1 bomber and now opposes the neutron warhead. Porter is one of the few GOP candidates who made defense even a tangential issue.

Speaking recently to the Wilmette Rotary Club, Mikva preached fiscal conservatism. He called lower capital-gains taxes "something I could support," called for getting "the handle on federal spending" and quoted the late Paul Douglas as saying, "You don't have to be a wastrel to be a liberal."

The polls say it isn't working. The neck-and-neck standing is less important than the trend in Porter's direction. So, Mikva now contends that Porter is abandoning past liberal positions on abortion. ERA - and the gun issue. In a district where street crime is not a principal pastime, Mikva brought up gun control.

At a recent taped interview at NBC in Chicago, Porter reaffirmed his anti-hand-guns position. After the taping, an obviously distressed TV technician told him he is participating in an NRA phone bank against Mikva. Now he wondered if this could do any good with Porter also against handguns. "I'm afraid that's the way it is," Porter replied.

With district polls showing 80 percent support for gun controls, Porter was rejecting special-interest backing usually irresistible for Republicans in favor of broader support on economic questions. This race, at least, will measure the tax revolt's political impact.