Back at the New York delegation, another swirl of delegates and reporters gathered around Mr. Kemp, the trim, outspoken Buffalo Congressman, who offered the assessment that Walter F. Mondale, the Democratic Presidential nominee, was ''dropping like a stone'' in the opinion polls with his talk of tax increases.

Only 20 paces away, Lewis E. Lehrman, the conservative, multimillionaire New York drug store magnate, doffed his coat to chat with strolling delegates and show the red suspenders that were the trademark of his unsuccessful run for Governor against Mario Cuomo in 1982.

Each scene flashed the signal that for some leaders the Republican future without Ronald Reagan is already visible over the horizon. Their jockeying here is more striking and purposeful this year than that of their predecessors in 1972, when the Republicans last assembled to renominate a sitting President. A Four-Year Gantlet

For, in the intervening years, it has become conventional political wisdom that a run for the nation's highest office is at minimum a four-year gantlet. The present field knows that Mr. Reagan laid plans for 1980 after his unsuccessful bid in 1976 and that Mr. Mondale gave orders to his lieutenants for the 1984 campaign within days of the Democratic defeat in 1980.

Indeed, the Republican competition here seems even more pronounced than among the Democrats in San Francisco, in part because Mr. Reagan can serve only one more term while Mr. Mondale's future is much less defined.

Realistically, the Reagan convention impresarios have given the party's leading lights a chance to showcase their talents - Mr. Baker on Monday night, both Doles and Mr. Kemp tonight.

Vice President Bush, who has to wait for a formal appearance with the President on Thursday, reached for his share of the limelight this afternoon with a well-timed convention hall news conference to top a round of appearances before state delegations. Tactics Dictated by Roles