An important new chapter in the education of Ronald Reagan as presidential candidate unfolded here Wednesday when he zeroed in on the "Carter depression" at the urgent advice of old pro, four-termed Ohio Gov. Jim Rhodes.

Deleting a quotation on military strength from the late Gen. Douglas MacArthur, Reagan cleansed his text for the Ohio Teamsters of virtually everything not directly tied to the nation's economic crisis.

To evoke MacArthur would have backtracked Reagan into the same quicksand of Vietnam and China he has spent two weeks trying to escape. The "Carter depression" moved him for the first time since the Republican presidential convention to promising dry ground for waging a successful campaign against the president.

One lesson to be drawn from the intervention of Rhodes is this: access to Reagan by canny, vote-hunters of Rhodes' caliber in the all-important hunting ground of the industrial Midwest can be decisive on Reagan. Economist Alan Greenspan, a key Reagan adviser, was not even consulted about the eye-catching "depression" terminology and disputed its accuracy except in certain parts of the country.

Even to consider invoking Gen. MacArthur's name in this recession-torn state with a 10 percent jobless rate would seem to defy common sense. Yet, just such lack of common sense has gogged the early innings of Reagan's campaign. His toughly worded attack on Carter economics has now given him the offensive on an issue ripe for exploitation.

Signs of party discomfiture have been coming in loud and clear, demanding that offensive. At a closed-door meeting of Reagan surrogates in Washington on Tuesday, Maryland Rep. Bob Bauman, a conservative hard-hitter, demanded of Reagan campaign manager William J. Casey: if we're supposed to be speaking for Gov. Reagan, you have to let us in on the takeoff.

A similar complaint was voiced by Rep. Sam Devine, Reagan's Ohio campaign chairman. He told an applauding House Republican Policy committee this week that Reagan's path to the White House has only one direction: throught the "disasters" of high inflation, interest rates and unemployment. After Reagan's Teamsters speech here, Devine confided: "I think they're waking up."

Republican straegists in Ohio, a state Carter barely carried in 1976, and in recession-rocked Michigan, Wisconsin, Illinois and Pennsylvania are convinced the election will be won or lost in the steel and auto mills of the industrial heartland. As Rhodes told Reagan: "Everything is unsettled today. It's all out there to be had."

Following Rhodes' earlier lead, the governors of these five states elaborated the message for Reagan here Wednesday evening. They warned that the Carter strategy of painting Reagan as a dangerous cold warrior has been more successful since the Democratic convention at Madison Square Garden than Carter could have hoped for. This August setback when voters' minds are on Labor Day vacations, not the presidential campaign, can be erased -- but, Reagan was told, only if he attacks, attacks and attacks again on inflation, unemployment and high interest rates. "Our people are angry and they are going to gang up on someone in this election," one governor told us. "Reagan has to make Carter the target."

The unanswered question is: does Reagan have the fortitude to follow this prescription? The major changes he agreed to write into his Teamsters speech cheered even skeptical Jim Rhodes. In addition to hanging "Carter depression" on the president, Reagan for the first time tried to personalize the economic issue. "After four years of Carter's little pills," he told the Teamsters, "ask the steelworkers in Youngstown if they like the way things have changed; ask the unemployed auto workers in Cleveland and the rubber workers in Akron."

Getting voters to ask those questions, and demanding answers from Jimmy Carter, could relieve him of that excess baggage accumulated in China, Taiwan, Vietnam and other areas far from the minds of the Ohio Teamsters and their blue-collar buddies.