Sen. Fritz Hollings, tall and silver-haired and unreasonably erect, resembles a Confederate cavalry officer imagined by the kind of novelists who write steamy romances with titles like "Gallop to Destiny" and "Shiloh Sweetheart." He is tall cotton in South Carolina, and in Washington last week he galvanized discontent about the administration's budget.

A president finds it tiresome when he sends a budget to Congress on Monday and on Tuesday an opposition senator begins gathering bipartisan support for a comprehensively different approach. However, Ronald Reagan can take comfort from the fact that Hollings' proposal demonstrates how far America's center of political gravity has shifted.

Hollings is a centrist Democrat who favors carrying over major portions of the fiscal 1982 spending and revenue policies into fiscal 1983. Using Congressional Budget Office premises, he says his proposal would cut Reagan's fiscal 1983 deficit by $115 billion, leaving a deficit of $42 billion. The out-year savings would be $169 billion in fiscal 1984 and $212 billion in fiscal 1985, when he calculates a $4 billion surplus.

"Reaganomics," he says, "is well-conceived." But too much was tried too soon. The economy got indigestion because the individual tax cuts were not delayed two years until the business stimulus took effect. By skipping the 10 percent cut scheduled for this July, and reducing to 5 percent the 10 percent cut scheduled for July 1983, Hollings would relieve the government of the need to borrow an additional $44 billion (at 14.5 percent).

>By skipping one year of cost-of-living adjustments for Social Security and other entitlements, he would save $24 billion. By capping cost-of-living adjustments (COLAs) below the consumer price index advance, he would get large out-year savings. He notes that Social Security was not increased under the New Frontier or in most years of the Great Society, and that the automatic COLA did not begin until 1975.

By freezing defense in 1983 and having only 3 percent real growth the next two years, Hollings would save $19 billion, then $30 billion, then $34 billion. He has a long record of supporting defense efforts, but believes that unless defense is also frozen, the attack on defense might go even deeper. By not exempting defense, his plan preserves its clarity but maximizes its challenge to the president's priorities of tax cuts and defense increases.

There is extraordinary nervousness, especially among Republicans, about the projected deficits. But whatever chance there is of passing Hollings' plan or something derivative (a defense freeze would be intolerable) depends on keeping it "clean." Otherwise it will become a bill weighing 100 pounds, with 99 pounds devoted to "fairness" exceptions for potent interests. This would dissipate the impression of fairness that derives from the plan's bold indiscriminateness.

Never has a party wrung so little political advantage from so much opportunity as the Democratic Party has wrung from the Republicans' many difficulties. This is, in part, because the most visible Democrats (Tip O'Neill, Walter Mondale) are the sort of unconvincing figures the Republicans would choose were they allowed to select their most conspicuous critics.

>Hollings, however, has been around long enough to know the game, but has not been an oppressive presence in the nation's living rooms. He was a governor for four years and has served 16 years in the Senate, where he is 16th in seniority (14th among Democrats). Now, with an exquisitely timed stroke, he has put a fox among the Republican chickens. At a moment of maximum skittishness, Republican senators are offered a plan that says: Reagan is a big spender--$115 billion too big in 1983 alone.

Meanwhile, Reagan resembles the Confederate officer who, at a dicey moment in Manassas, turned the tide, and acquired a nickname, by "standing like a stone wall." A president accused of "intransigence" probably is acting presidential. Being president involves resisting the eddies of passion and panic that can roil the surface of a shallow lake like Congress. Being Ronald Reagan involves being more compromising than casual observers understand, because the compromising occurs within a forcefully defined framework of basic priorities.

The heels of cowboy boots are splendid for digging in, and he must dig in regarding defense. Eventually--at an action-forcing deadline such as the debt-ceiling vote coming in late spring--he can say: let's compromise. I'm immovable on defense, but I'll move several inches on taxes and as far as Congress has the courage to go with Hollings' proposal for a one-year moratorium and then a cap on the entitlements' COLAs. Either he will have bipartisan support for something difficut but desirable, or he will have called a lot of bluffs--those people jumping on Hollings' bandwagon will jump off.

Reagan may have underestimated the degree to which deficits threaten the glue--belief in balanced budgets--that holds Republicans together. But regarding the heart of the matter--entitlements--the first serious challenge from a Democrat pushes Reagan to "retreat" in a direction he probably wants to go. That demonstrates how thoroughly he has done what few have done: he has re-written the political agenda.