In proposing the large new cuts in Social Security that he did last week, Ronald Reagan plainly made the first serious political blunder of his presidency. The reaction was such that, within 10 days, he was forced to retreat.

But Reagan's proposals may still turn out in the long run to have at least as many pluses for him as minuses. He will take some flak. He is also now likley to get much larger cuts in Social Security than anyone believed even two weeks ago.

As critics have been proving, it's easy to recite the negatives.

The White House, fresh from its budget victories, was too confident. The plan was put together too hastily. Strategists were to preoccupied with assuaging the financial markets, too little concerned with likely reaction in Congress. Reagan lost momentum; the Democrats were able to regroup.

But all that may be short-run. While the damage now is politically serious, it's also possible the impact will fade soon and over the longer-run the administration may reap some visible pluses. Consider this line of reasoning:

The president and some of his advisers have always wanted to trim back the Social Security program -- partly on grounds that its benefits formulas had grown too generous -- and what better time to propose it then when the system once again is financially in trouble?

What better time to propose such cuts than in a president's first year in office -- particularly now, when Reagan is riding high in the wake of his budget victories. Jimmy Carter, for example, also proposed painful Social Security measures -- higher taxes -- in his first year. He also unveiled his energy program that year, knowing it too would be unpopular.

Political flak is inevitable when anyone recommends trimming back the Social Security program, no matter what the merits of the proposals. The giant program's benefits now go to one American in seven. Sure, the initial reaction has been a firestorm. But the flames are abating, and the pain may not last for long.

When all is said and done, Reagan almost certainly will wind up wringing deeper cuts out of Congress now than he would have had he not gone out on a limb this way. A House subcommittee headed by Rep. J. J. Pickle (D-Tex.) had already been working on an overhaul package of its own. But who had heard of the Pickle plan? It was Reagan who energized the issue.

Moreover, the move has goaded Congress sufficiently that whatever bill emerges is more likely to be bipartisan, rather than forcing any one party to hear the brunt of the blame for the changes. Lawmakers know they must face the Social Security problem soon, or the system will go on the rocks.

Whic scenario will prove to be correct? The optimistic one, the pesimistic one, or both?

With a few flame-covered timbers still falling, it's realy too early to say for certain. There's no doubt the plan has emboldened some Democrats who previously might have been too much in awe of the president to oppose him very much.

But as the days go on, many observers, both Democrats and Republicans believe the president will emerge with little serious long-run damage. Even key Democrats concede the setback is unlikely to have much impact on Reagan's tax-cut and budget proposals.

At the same time, it's clear the administration has learned a valuable lesson from the fracas. As Murray L. Weidenbaum, Reagan's chief economist, conceded this week, "Hindsight tells us that we would have been better advised to spend more time discussing our views with . . . the Hill."

How did an administration that was credited with being so politically savvy get into this kind of bind?

The short answer -- obtained from a series of interviews with White House officials and other strategists -- is that even savvy administrations can get into trouble, particularly when they're so caught up in other issues that they fail to look at their proposals realistically.

Here, from talks with White House officials and others involved in the Social Security decision, is an account of how it all happened:

The proposal was put together mainly by White House domestic advisers, incredibly, without much consideration for the political reaction the plan was likely to ignite.

Top Reagan political aides weren't fully brought in "until it was too late," one insider says. Another adds that everyone was so preoccupied with the budget and tax programs that no one really thought it all through.

Strategists were too closely intent on assuaging the financial markets to see the pitfalls in the timing of the Reagan package.

The markets were calling for still more cuts in spending to hold the budget deficit down and the finanically ailing Social Security program, roughly a fifth of the budget, seemed a natural.

The Ways and Means subcommittee headed by Pickle already was drafting its Social Security plan, and the White House had promised Pickle and Senate budgetmakers it would quickly send up new proposals.

Critics says the White House could have parried both these demands with a stopgap proposal, such as temporary borrowing from other government kitties. But the Reaganites apparently were convinced they had to move more broadly.

Presidential counselor Edwin Meese III, one of Reagan's most politically astute senior advisers, was quickly briefed on the proposals o May 9, but there was little time for serious objections, and Meese apparently raised none.

Two days later, the plan was presented formally to Reagan and his key economic aides. Insiders say there was about an hour of discussion, but Reagan, though no neophtye politician himself, didn't question the politics.

For the moment, interviews on Captiol Hill indicate Reagan certainly hasn't helped himself politically by unveiling his Social Security plan when he did.

But the jury is still out on what the long-run impact of his move will be.