Nobody paid much attention the last time the Sheet Metal Workers union held a convention.

This time it's different. Sen. Edward M. Kennedy (D-Mass.) opened their gathering here this morning, to be followed by no fewer than six other potential 1984 presidential candidates as the week goes on.

A circus of TV cameras, several dozen reporters and assorted political groupies were in attendance along with the 650 union delegates.

Sen. Robert J. Dole (R-Kan.), representing the Republicans, made what was billed as his first address ever to a labor union.

The Sheet Metal Workers sudden magnetic appeal is the result of an ambitious plan approved by the AFL-CIO on Aug. 5 for organized labor -- for the first time -- to endorse a presidential candidate before the first presidential primary or caucus, possibly as early as December, 1983.

The plan has transformed the labor movement from a forlorn wallflower to the belle of the political ball this season. And it has set the candidates to wooing the unions like lovesick swains fully two years and two months before the 1984 presidential election.

Kennedy, one of the early favorites of many labor leaders for the Democrat's presidential nomination, told the union that President Reagan's economic policies had devastated the economy, thrown more than 3 million Americans out of work, and forced many others to live under the poverty level for the first time in their lives.

"They said there has to be belt-tightening and fiscal responsibility but there's no reason why the poor and the elderly should have to bear the brunt of these policies," Kennedy, who was the Sheet Metal Workers' choice over Jimmy Carter in 1980, said.

Kennedy said his appearance did not mean he had decided to be a presidential candidate again, and emphasized that he is primarily concerned with being reelected to the Senate in November.

The endorsement plan, spearheaded by AFL-CIO President Lane Kirkland, its author, still faces many obstacles.

For instance, Edward J. Carlough, the politically active president of the Sheet Metal Workers, said today he doesn't like the idea of endorsing a candidate before the first primaries.

Douglas Fraser, president of the 1.2 million-member United Auto Workers, has also expressed reservations about locking the labor federation into a definite time for an endorsement rather than maintaining flexibility.

One of his concerns is that labor leaders should not be forced to choose between two old friends such as Kennedy and former vice president Walter F. Mondale, if they are running neck and neck, and risk splitting the labor movement.

Fraser, Carlough and others also want to give "new faces" a chance to show their stuff in a few primaries, they say.

Still, many, including Carlough, believe that organized labor has been in the political shadows for too long and that some sort of unified effort is in order.

"Lane is moving in the right direction," Carlough said.

The Sheet Metal Workers' favorite politician is said to be Sen. Henry M. (Scoop) Jackson (D-Wash.). Carlough supported his candidacy in 1976.

Meanwhile, many unions find themselves swept up in a marathon political whirl.

"In the past, it's been difficult to get people to speak at our conventions, but now we're beating them off with a stick," said Glenn Watts, president of the Communications Workers of America, recently.

The choice of a speaker has taken on a political significance.

If somebody is invited to speak, will people think they've been annointed?" Watts wondered. "Next year, what do you do? Have them both? How do you invite only one? Our people are in love with both of them?"

There are about two dozen union conventions and state labor gatherings, large and small, in the next three months alone.

By sometime next summer, it is up to each of the 99 unions affiliated with the AFL-CIO to work out a procedure for determining which candidate to endorse. Some will take a vote. In others, the union leadership will decide based on a more informal pulse-taking.

Because two-thirds of the federation's 15 million votes are required for endorsement, a handful of the biggest unions could unite and deny the prize to any candidate.

On the other hand, about a dozen unions, including three or four big ones, could swing the endorsement for somebody, federation spokesman Murray Seeger said.

In addition to Kennedy, Dole and Sen. Ernest F. Hollings (D-S.C.) today, the schedule for the next four days at the Sheraton Centre includes: Sens. Alan Cranston (D-Calif.), Daniel Patrick Moynihan (D-N.Y.), John Glenn (D-Ohio) and Mondale.

Sen. Gary Hart (D-Colo.) was invited but could not attend because of scheduling problems, Carlough said. Dole was invited in his place because of his key role in getting Reagan's tax bill passed.

Dole was received warmly--but not as warmly as Kennedy, who was presented with a contribution from the union's political action committee.

"The only union that has supported me in the past," Dole told the delegates, "is the Women's Christian Temperance League -- and they're having second thoughts."

The Senate Finance Committee chairman repeated his appeal for a special post-election session of Congress to deal with the explosive issue of Social Security.