In trying to convince Americans that what's good for the Pentagon is also good for them, the White House is haunted by the symbolic political success of President Reagan.

The president, who understands the axiom that a salesman has to value his product before he can persuade others of its merits, has sold himself and his countrymen on the proposition that he has restored U.S. military power after a long hiatus. He has a good case, though not an overwhelming one. The military buildup he celebrates began under President Carter, who may have been partially impelled by Reagan speeches mocking Carter's belated realization of Soviet motives after the invasion of Afghanistan.

Reagan's exploitation of Carter's naivete and the frustration Americans felt in 1980 about the failed rescue of the hostages in Iran helped make U.S. military spending fashionable for the first time since the Vietnam war.

This public mood gave Reagan a boost in his first term. While failing in his avowed goal of modernizing U.S. strategic forces because he refused to accept Carter's sensible plan for deploying the MX missile, Reagan persuaded Congress to spend an extra trillion dollars on defense. Now that deficit reduction has become a central goal, it is the perception of Reagan's success that imperils his defense budget.

White House polls show that voters believe the Pentagon has been fed while domestic programs have been starved. These voters overwhelmingly oppose giving the Pentagon another blank check. On Wednesday, Reagan will try to swim against this tide with his favorite communications contrivance -- a nationally televised speech from the Oval Office. But in seeking defense spending increases, Reagan will be struggling to overcome public sentiment that has changed radically since 1980.

Reagan's managers realize the problem. They will try to solve it with a speech that ignores budget numbers and contends that the United States must counter Soviet regional efforts in Afghanistan, Angola and Central America and spend more on defense to induce the Soviets to become serious about arms control.

Reagan believes that the summit meeting he held with Soviet leader Mikhail Gorbachev in Geneva last November and the repeat event planned this summer in Washington is a product of the U.S. defense buildup. The administration argues that the promise of an agreement limiting or eliminating medium-range nuclear missiles in Europe would be jeopardized by a defense cut.

All this may be true. But Reagan and his advisers have undermined their arguments in the sloppy and confused way they have presented the case.

In his State of the Union address the president appealed to Congress for a "bare minimum" increase in defense spending and said, "My budget honors that pledge." But when the budget was published, a promised 3 percent increase turned out to be 8 percent. Key legislators in both parties charged that the actual increase was even higher, perhaps 12 percent, and said that projected defense spending had been underestimated by $15 billion.

The administration performance in behalf of its private request to congressional leaders for a $100 million package for the Nicaraguan rebels, including $70 million in covert military aid, was even more dubious. Why can't the president appeal openly for a proposal that he believes vital to the national interest? Instead, Reagan allowed his director of central intelligence, William J. Casey, to impugn the motives of opponents to this aid package by distributing a document suggesting that opposition to aiding the rebels was Sandinista-inspired.

I do not believe that Reagan intended this implication, for he has a long record of honorable political combat. But the tactics of his subordinates introduce an ugly note into the debate on aiding the Nicaraguan rebels, and suggest that the case on the merits is rather weak.

We have learned, over the years, not to underestimate Ronald Reagan. He may overcome the glitches in his sales campaign with a brilliant speech Wednesday, and Republican leaders on Capitol Hill may succeed in removing the sour taste of the CIA smear of the opponents of the contra-aid package. But in a critical sixth year of the Reagan presidency, the White House is off to an unpromising start in its latest military spending campaign.

Reaganism of the Week: Asked in Grenada last Thursday how he felt at the scene of his "greatest military adventure," the president replied: "I didn't fire a shot."