The mark of Ronald Reagan's possible maturity as presidential candidate is found in the new contradiction between what he feels privately and what he says out loud, which up to now have been one and the same.

In confidential chats with top staffers and close friends, Reagan insists that during those 17 days of blooperism he was more right than wrong; but in public, he now accepts admonitions such as this from one worried staffer: "Remember, Governor, not a word about China."

Thus, Reagan curbed an instinctive reaction to rap President Carter for agreeing to the sale of high technology to Communist China. The trend toward self-discipline coincided with Carter's tribulations in the great debate over the debates, giving Reagan's tour of the crucial Great Lakes states last week the look of a much-needed turning point.

"I think we've finally got the rhythm," a key staffer remarked to press secretary Lyn Nofziger in midweek. The rhythm is leisurely but tightly controlled. Reagan's slow-paced tour from Cleveland to Buffalo to Erie, with a maximum of two or three events a day, and on to a long quiet weekend at his rented Virginia farm in the Middleburg hunt country is a radicle departure from the usually frenetic pace of presidential campaigning.

Out of it came what Reagan's political aides want: evening and morning network coverage showing Reagan chopping away at Carter's economic record. From Cleveland came an attack on the "misery" of the president's energy programs; from the empty Lake Erie port of Buffalo, where unemployment is 10 percent, a warning that U.S. ships now carry only 5 percent of America's ocean trade; from Erie, before a large, friendly crowd in the courthouse square, a challenge to Carter "to stop running away from his record" on the economy.

That rounded out a week starting with the Chicago speech reaffirming Reagan's support of Kemp-Roth tax cuts (a speech that brought an unexpected congratulatory telephone call from Dr. Arthur Burns, the skeptical economic eminence of the Republican Party).

There is optimism in the Reagan camp that Carter has trapped himself in a non-win position on the presidential debates. More important than a debate windfall for Reagan, however, is whether his new-found discipline can be enforced. One Reagan staffer, pointing to Stuart Spencer sitting across the aisle from Reagan in the front compartment of his campaign jet, exuded confidence. "That's what Stu's for," he told us.

Spencer is the political operative who handled Reagan's first campaign for governor in 1966 but signed on as Gerald Ford's chief tactician in the 1976 presidential campaign. His influence within the Reagan camp was peripheral -- until the China blooper, the first of a series, burst on the political horizon Aug. 17 and Spencer was soon detailed as Reagan's watchdog.

Spencer's qualifications for playing watchdog are threefold: political experience, love of combat and boldness. "If Social Security comes up," a Reagan aide told us, "Stu will be there to tell Reagan, 'Your're for it, Governor.'"

There are, however, bounds beyond which Reagan should not be curbed: his sometimes instinctive sense for clever one-liners that get the crowd roaring. Now part of the standard litany is his description of the economic crisis: a recession is "when your neighbor loses his job," a depression "when you lose your job." Reagan added the third step, unrehearsed: "And recovery is when President Carter loses his job."

Reagan's reaction to Carter's refusal to participate in the first three-way debate with Reagan and Rep. John Anderson hit the right note: gentle riducle. Carter, he said, might have to learn about debates the hard way, as Reagan himself did when he refused to debate before the Iowa caucuses last January and lost to George Bush.

This is the Reagan his handlers want to advertise: hard-hitting on the president's handling of the economy; gently ridiculing on the president himself; distant from China. With 50 days left before the election, Reagan hit that stride last week in the states he must win, but in a presidential campaign, 50 days are an eternity.