The Republicans have nominated an aging former actor and a roving Texas Yankee to head their 1980 ticket, but the future of the Grand Old Party lies with a group of young men who played only supporting roles at the extravaganza here this week.

Former pro football quarterback Jack F. Kemp, a congressman from New York, by displaying skill in the GOP platform hearings and by giving one of the best speeches of this convention, solidified his position as the standard-bearer for this group, many of whom hold office in Washington.

Reps. David Stockman of Michigan, who also played a key role in shaping the platform, Newt Gingrich of Georgia and Ed Bethune of Arkansas are others whom Kemp considers "activist" allies. They get part of their politcal "lift" from the leadership vacuum created in the party by the Nixon scandals.

The proposition they are pushing is that the Republican Party should be looked to as the party of activism and populism, the party of tax cuts for the worker, of full employment, a strong defense and less government.

The Democrats have become the party of elitists, of the status quo, passive," Kemp said, hammering on the GOP's favorite anvil -- the Democrats' policy of curing inflation by increasing unemployment.

Ronald Reagan has not only embraced this GOP approach, but also is giving it a bear hug. It is part of his standard speech and is embodied in the party platform. But how far will he take it?

"I'm going to spend the rest of the year campaigning for Congress and for Reagan -- and also make sure there is strong voice within the overall Republican campaign to make sure we keep the ideas in the platform at the forefront," Kemp said.

Kemp, with younger Republicans Sen. Richard Luger of Indiana and Rep. Guy Vander Jagt of Michigan, was among the runner-ups in Reagan's presidential contest. They claim they came out winners even though Reagan selected someone else.

"Our star really went up," said an aide to Lugar. "This brought an incredible amount of publicity to a guy who wasn't all that well-known." Indeed, until the vice-presidential playoffs, Lugar's main claim to fame was as "Nixon's favorite mayor" when he held that post in Indianapolis.

Kemp and the young Republican go-squad might have been better off in terms of political ambition if Reagan had gotton former president Gerald Ford on the GOP ticket. With both standard-bearers nearing their 70s -- Reagan is 69, Ford just turned 67 -- the party would have been up for grabs in the 1980s.

George Bush, 56, a survivor of Watergate, presents an obstacle that could mean, in the words of an aide to right-wing Sen. Jesse Helms of North Carolina, "another bitter struggle for the heart and soul of the party in 1984."

Others disagree. "The real victory for the New Republicans was back before, when Ronald Reagan said 'I understand your economic proposals and I'm going to accept them,'" said David Warnick, an aide to Gingrich.

He was referring primarily to the controversial tax cut proposal and other measures embodied in what Kemp calls "incentive economics" and espoused in the Kemp-Roth bill, which Kemp cosponsored with Sen. William V. Roth Jr. (R-Dell.).

Critics of the approach -- including Bush earlier this year -- have called it irresponsible and inflation-causing. Supporters, including Reagan, insist that it would revive the staggering U.S. economy.

Over the long haul, Reagan's selection of Bush over Ford "forces the New Republicans back to the trenches, so to speak, back to where they should be," Warnick said, "in the parliamentary body of Congress. . . They had been relying on Kemp and personality rather than organization and thoroughness."

In the lexicon of the New Republicans, Bush is an Old Republican. "Whether he's a moderate or conservative is irrelevant," Warnick said.

Kemp, who turned 45 this week, has been criticized by some Reagan aides as "too rah-rah, plastic and green" to be a heartbeat away. And the former Buffalo Bills quarterback turned economics buff suffered from a Johnny-One-Note image as a cheerleader for his pet economic theories.

One of his goals at the convention was to dispel that image, according to an aide. He took special pains to become involved in defense and other noneconomic issues during the platform hearings, and to display a conciliatory, consensus-building image.

Still, he could not resist injecting a phrase about "shattering the Keynesian consensus" into a short pep talk to a group of admiring college-age Republicans this week.

Supporters, however, are convinced that Kemp's performance has dispelled much of the image problem.

At his disposal, as he seeks to broaden his base, is a political action committee that finances his activities, such as speaking engagements, and provides a mechanism by which he can collect political credits.

Kemp declined to speculate on the possibilities for 1984, but conceded that having Bush as vice president would be a blow to his own political ambitions -- "if getting elected president in 1984 were my top priority. But it isn't."

A Kemp aide, however, later said that the congressman's speech to the delegates Tuesday night got a reaction not unlike that of "Kennedy in 1956, or Reagan in 1964."