When the Democratic presidential nomination debates begin, it's going to be great to have Sen. Ernest F. (Fritz) Hollings of South Carolina around. He will, it's safe to predict, keep things from descending into dull unanimity.

Hollings has a reputation around the Senate as a smart, tough, acerbic fellow who suffers no excess of modesty. He has been voicing his dissents from the conventional Democratic wisdom ever since the party's mini-convention in Philadelphia last year. They are getting sharper all the time.

Last week, he was up on his feet during the Senate's budget debate, literally crying "shame" on his party. His goal, he said, was to "shame them (his fellow-Democrats) into doing something right" on the budget. The right thing, as he sees it, is to freeze domestic spending, halt further tax cuts and limit defense hikes to the 3 percent real growth target of the NATO alliance, to get the deficits down.

His argument was couched in blunt political terms. "The American people know we are the party of education and consumer protection and equal opportunities for all Americans regardless of race or religion," he told Senate Democrats, "the party which seeks to end poverty, which ardently desires an end to the nuclear arms race, and which cares most deeply about the environment.

"The American people share those values with us. They are 100 percent with us on those issues. If elections were decided on those issues alone, we would never lose an election. And yet the American people have taken power away from us in the U.S. Senate and in the White House."

"Why?" Hollings demanded. "The answer is obvious. They have lost faith in our ability to manage the economy. They have lost faith in our willingness to balance our checkbook."

While deriding President Reagan and the Republicans for their tolerance of huge deficits, Hollings cried "shame" again and again at Democrats who "deliver the same old speeches calling for more and more federal spending without telling anyone where the money is going to come from."

Hollings didn't get many votes for his freeze-- 16 to be exact--and none from any other presidential hopeful. He may not get many votes in the primaries, if today's polls are at all predictive. But his party is beginning to realize it cannot ignore the warning he has been raising.

The most certain area of vulnerability in Reagan's record in 1984 will be the massive deficits he has accumulated--$700 billion in four years by the administration's own January estimate, more by most other calculations. Unemployment may go down and the business cycle go up by November 1984, but there is no question Reagan will have to explain the worst flood of red ink of any peacetime president in history--a flood many conservative businessmen say will engulf our long-term economic future.

But if the Democrats have no greater credibility on that issue than Reagan, as Hollings insists, then they pose no political threat to the president.

Do they deserve such credibility? Probably not. Looking back at 1981, when the budget and tax decisions that opened the floodgates of deficit spending were taken, one is struck by the failure of congressional Democrats to draw a sharp line with Reagan.

The Senate Democrats, still in shock over losing their accustomed majority status, offered no comprehensive alternative on either the budget or the tax bill. On final passage of both measures, most of them supported Reagan's policies. Individual Democrats' amendments to restrain the military buildup or hold off the tax-cutting revenue losses rarely drew more than half the Democrats' votes. 2 In the House, where Democrats retained a nominal, if not working, majority, they fatally compromised their position. After coming forward with a responsible plan, the Budget Committee majority quickly abandoned it, in pursuit of conservative votes, by upping its defense spending target to match Reagan's. That decision helped launch the dizzy escalation of Pentagon spending that Congress is still struggling to restrain.

The second--and worse--Democratic error in the House was starting a bidding war with Reagan to see who could put the most "sweeteners" into an already overweight tax-cut package. Some of the most egregious revenue giveaways were suggested by the Democrats, who happily went along with the Republican strategy of cutting taxes for the big guys first and most, and for the little guys less and later.

Hollings' own record has been consistent. Back in 1981, when it was hard to do, he voted to slow the spending pace and to limit the tax cuts. He was one of only nine Senate Democrats to oppose passage of both the Reagan budget and the Reagan tax plan.

The Democrats may well reject his candidacy, but they can hardly afford to ignore his warning.