President Reagan revived the military strength issue this week, but it has taken on a different shape from the one it had in 1980, and most of the advantage in the change seems to fall to Walter Mondale.

From the late Carter period on, few Democrats were prepared to resist the general appeal for more power, meaning to most people -- including key administration figures -- chiefly a higher defense budget. But in the late Reagan period, even mainstream Republicans are gagging on the budget numbers and on the ever-growing contribution they make to the gigantic deficits which seem to be the No. 1 economic concern of everyone in America except Ronald Reagan.

When Soviet tanks rolled into Afghanistan, no one was too fussy about spending quickly in order, in some generalzed psychological sense, to "catch up" with the Russians. With the Russians now plainly bogged down in Afghanistan, the earlier fear of a Red juggernaut moving relentlessly into the Persian Gulf has dulled. Awareness verging on anger has spread that the hasty green light given the Pentagon is allowing it to waste billions of dollars -- tens of billions? scores? -- through bad planning and procurement.

This is not to say military strength has been devalued as a political asset for Ronald Reagan, who obviously still believes in power. So, the polls tell us, do a majority of Americans. Aware of their party's vulnerability on the issue, his Democratic opponents never fail to pay homage, as Defense Secretary Caspar Weinberger puts it, to a strong defense. (It is an enduring puzzle, by the way, that no one seems to recall that the deepest defense cuts of the 1970s were administered by Weinberger, then Nixon's budget director and now the Reagan administration's toughest critic of Democratic defense spending.)

The bravest that most Democrats have been is to assail the administration on the somewhat esoteric grounds of military readiness. Readiness is, like bad planning and cozy procurement, a legitimate issue, but one that tends to be used by liberals who feel critical but are reluctant to say outright that the budget is too high.

The actual budget changes of which Democrats speak go merely to a modest decline -- a percentage point or two -- in the rate of increase. Nobody in national politics is yet claiming that it is safe, or not only militarily safe but economically essential and administratively prudent, to top off the Pentagon budget at the level of the historic new peak achieved in six years of Carter and Reagan increases.

In the fiscal year just beginning, the total defense budget emerging from Congress provides about $293 billion. The apples-and-apples inflation-adjusted figure for 1979 was $195 billion: an immense, built-in increment of $100 billion 1984 dollars.

At the least, or so I would hope, the next Congress (if not the next president) is going to proceed very slowly before moving into the big-bucks stage of any major new program. One such would be Reagan's multi-billion-dollar ballistic-missile defense, known derisively -- though, note well, repetition dulls the derision -- as Star Wars.

Reagan could increase defense and take a hard line in this term because he made a plausible case to the people for "rearming America" and "standing tall." Most of those who suspected he was going to excess held their fire or sniped from safety. They were disarmed by the sequence starting with the Soviet power play in Angola in the 1970s and leading to the American humiliation in Iran and the crushing of Poland's Solidarity. They were ready to see what the Reagan-type application of power could bring.

The results, you could say, are mixed: no successful new Soviet power plays buto successful American power plays either, except for Grenada. The voters will be pronouncing their judgment, but already it is evident that Reagan is somewhat vulnerable to his own relative success. No matter who is elected, the large issue in the next term will not be the amassing of further power. It will be the conversion of power into greater global stability, in arms control with the Soviet Union and in regional conflicts.

If strength was the standard in 1980, strength plus subtlety is the requirement now.

Some of the people who know Reagan best from the old days say he was a first-class negotiator for the screen actors' guild. The rest of us have to take it on faith. Nothing yet visible in his conduct of American policy demonstrates he can run the difficult passage of creating a sensible, coherent bargaining position within his own government and converting it into a diplomatic agreement. By philosophy and temperament, Mondale is inclined that way, but of course he has not been tested by fire. This is, in foreign policy, what the 1984 election will be most about.