The Democrats are demonstrating in New York today what the Republicans showed last month in Detroit: the national convention, as now constituted, is in danger of becoming the mastodon of American political life. It is not yet extinct, but it deservedly stands on the endangered-species list.

There is no mystery about its imperiled state. It has been overtaken by political reforms -- most notably the increasing numbers of primaries that virtually guarantee who the presidential nominees will be long in advance -- and changing times. What the Democrats have added is evidence that nearly everyone, delegates and ordinary citizens alike, is aware of how irrelevant the old-style convention format now seems.

Within minutes after Monday night's crucial roll call assuring President Carter his renomination, Madison Square Garden was emptying, and the delegates, even those for Carter, were expressing a sense of letdown. It was all over scarcely before it began; no political air ever left an overinflated convention balloon so swiftly.

At the same time, the public was registering its verdict on the convention. As with the Republicans in Detroit, the television ratings rated the Democrats' show a flop. Despite all the publicity attending the opening session, with its decisive floor actions scheduled to unfold, many more American television viewers tuned to something other than the convention.

Here in New York, the nation's major population center, only 33 percent of those watching TV tuned to network coverage of the convention. In the other two largest areas, Chicago and Los Angeles, only a minority also tuned in to watch the Democrats. The figures were 42 percent for Chicago and 45 percent for Los Angeles.

Those low ratings continued the downward trend reflected by the public reaction to the Republicans last month. The first day of the GOP gathering showed a sharp drop in the number of people watching, compared with the Gop audience four years ago.

The people are sending the politicians and the networks a message. They are not interested in watching the tedium of gavel-to-gavel coverage of a multimillion-dollar extravaganza whose outcome is preordained and in which little suspense remains.

Now, even many delegates are in agreement. "We ought to adopt the platform at a miniconvention and go to a national primary," said delegate Chuch Gifford of Newton, Iowa. Sala Burton of California, the wife of Rep. Phillip Burton, is attending her seventh convention, her third as a delegate. Asked if she thought conventions still serve a point, she said: "As of today, no. I know people who are going home. They think it's all over."

Her solution, and that of many others interviewed, is to "do something about the primaries."

Both of those delegates supported Kennedy here, and voted with his forces in the so-called "open" convention fight. But their larger concerns about the convention format also were being expressed on the floor today by Carter delegates who stood strongly with the president in that fight. t

Over in the Texas delegation, Carrin Patman was saying that she had participated in the Democratic reform movement for years, and voted with Carter on the rule binding delegates to vote for the candidate they were pledged to support.

But, she said, "I'm very disturbed. I think we've done something we didn't intend to do. We've turned over absolute control of the nominating process to the presidential candidates and, worse, to their staffs. People feel pretty disgruntled today. We tell them how important they are and how their participation really makes a difference. We get them puffed up and proud about coming here as delegates for the first time. And then they see they're not really that important at all at the convention itself. It makes them feel very disappointed."

Her suggestion is to streamline the convention process -- cut back on the boring, meaningless speeches and early-morn-to-late-evening sessions. It is a position that other delegates endorse.

Thomas C. Cronin, for instance, a political scientist at The Colorado College, agrees that the convention could be shorter. It might be good to have a flexible schedule, he said, "with time modules you can add or subtract as you need them. Maybe once in a generation you may need a day to debate one issue."

Cronin argued that the convention still has a function to perform. It permits a party to gather and debate issues, get national exposure and heal the wounds of party political strife before the fall campaign.

And certainly many here find the convention an exhilarating experience. Susan Bowers, 28, of California, a Native American delegate, is like the majority of these delegates attending her first convention. She has found it "very exciting. It's a thrill to be here. I've learned a lot. I'll go back a richer person."

Others are here as a testament to democracy. Mary Pearson of Fresno, Calif., is a middle-aged teacher. She teaches grade-school English. This year she decided to set an example of good citizenship for her students and ran for delegate. She raised most of her own money for the trip here. It's been a marvelous experience, she says, one she wouldn't have missed for the world.

But the words of another Californian, Jesse Unruh, the state treasurer and Carter leader, are typically more critical. In 1960, he said, the convention had "more input from people who had a continuing interest in the parties, which was healthy. What has changed, and what I object to, is the separation of the function of nominating the president from the rest of the activity of a political party. These delegates feel no responsibility for anything but the presidential candidate they're backing. Twenty-five percent of the Carter delegates and 55 to 65 percent of the Kennedy delegates we'll never see again."

None of these concerns addresses another side to the convention. They've always been a place for mixing foolishness and seriousness. Now, in the TV age, they've been transformed into a great gathering for assorted media groupies, where members of the press outnumber delegates by four or five to one and the expenditure of money mounts, it seems, even while the importance of the event diminishes.

This time, though, it seems that the public and politicians and -- dare we say it? -- even the press are having second thoughts about their respective roles.

Collectively, they're questioning whether this quadrennial exercise in excess is worth preserving or whether it should be relegated to the political museums to take its place along with the torchlight parades of the past.