After being introduced as "a man of conviction," Rep. John Anderson spent the next 40 minutes demonstrating to a College of Idaho audience that his clearest conviction as an independent candidate for president is that he considers himself vastly preferable to either Jimmy Carter or Ronald Reagan. But that seems enough for his new legion of admirers.

This was brought home when Anderson, fielding a serious question about the economy, fumbled it with an incomprehensible reply that could not possibly have satisfied anybody -- particularly the questioner. Yet after the speech, that questioner gritted his teeth and declared he still preferred Anderson to Carter or Reagan.

Traveling through California and Idaho launching drives for signatures to get him on the ballot, Anderson concentrated on denigrating Carter and Reagan. The content of his abandoned campaign for the Republican presidential nomination has all but disappeared. His famous 50-cent-a-gallon gasoline tax, once the cornerstone of Anderson's claim on courage, is mentioned only when a questioner brings it up.

Nevertheless, through his descent from issues to personalities, the tone of his speech is markedly more left-to-center than in Republican primary days. So are his audiences, even in ruggedly conservative Idaho. He was cheered in Boise for voicing support of the Panama Canal treaties, which are so objectionable in this state that they may condemn Sen. Frank Church to early retirement this year.

The nature of Anderson's current campaining was typified by his response to that economic question at the College of Idaho, posed by a young Boise lawyer who with his wife had driven 30 miles to Caldwell to inspect Anderson at close range. His question: considering simultaneous inflation and recession, are Americans disciplined enough to accept some unemployement while inflation is being curbed?

Anderson's answer waltzed past the question and instead discussed unemployment in terms that betrayed no familiarity with any known economic theory: since there is so much work to do in modernizing U.S. transportation and energy systems, "I just can't believe that we have to resign ourselves to a permanent slagheap of the unemployed." When another questioner smelled out in this the promise of massive government spending, Anderson shouted "No!", explaining that private business could help if given "proper kinds of incentives, through the tax code particularly."

But what "proper kinds of incentives"? Anderson has denounced the Kemp-Roth tax reduction that he passionately implored the House to pass less than two years ago. His position papers support faster tax write-offs for new equipment, but he seldom talks about it and lately has been suggesting ear-marked tax relief for stricken industries. Not long ago he proposed a wage-price freeze, but seems to be backing away because of slightly reduced inflation.

Signs of this jumble were found in his elliptical answers here, which disappointed the young lawyer from Boise. "But I'll still vote for Anderson," he told us. President Carter is out of the question; Reagan's economics may be closer to his, but he could never vote for anybody who seeks right-wing cheers by exaggerating the number of federal regulations governing stepladders.

Anti-Carter, anti-Reagan sentiment is stimulted by Anderson's switch from issues to personalities since coming under the tutelage of New York-based media wizard David Garth. White sniping at the president, Anderson reserves his best shots for Reagan. Having battled the Republican fight for the past decade, he delights in uninhibited assaults on its current champion.

Anderson seldom omits from a speech his claim that Calvin Coolidge is Reagan's "role mode." At the College of Idaho, he went further by noting Coolidge's two-hour afternoon nap as president, then adding, "I don't think the next president, despite the infirmites of age . . ." He was interrupted by sustained laughter.

Over Reagan-baiting is not the only evidence that Anderson, having shed all Republican pretensions, is more actively courting the left. Before he came to Idaho, he addressed Los Angeles Mexican-American leaders at Lucy's El Adobe restaurant, Gov. Jerry Brown's favorite hangout. After getting bogged down in illegal alien problems, Anderson attacked Carter's "hawkish mentality" in foregin affairs.

That began the day that saw Anderson praise the Department of Education, defend SALT II, reiterate support for gun control and, while professing neutrality, indicate his clear preference for Democrat Church over his Republican challenger, Rep. Steven Symms.

But Anderson spent 24 hours in California without mentioning his celebrated 50-cent gasoline tax and probably would have maintained the same silence in Idaho had not a local reporter in Boise asked him about it. Anderson endorsed the tax with a noticeable lack of enthusiasm ("I'm not prepared to abandon my idea"), then moved to more pleasant matters. John Anderson's major theme as he begins his independent candidacy is no longer a call for national but a reminder that he is neither Carter nor Reagan.