The end of 70 years of internal Republican strife seemed in sight when Thomas H. Kean, a blue-blood pillar of the eastern Republican establishment, appeared at a New Jersey statehouse press conference flanked by two tigers of the Republican right: Rep. Jack Kemp of New York and Jeffrey Bell.

Bell, a longtime Reaganite and Senate nominee here in 1978, supports Kean's front-running campaign for the Republican government nomination. Kemp was in Trenton to endorse Kean's tax-cutting package. Seated alongside them was Rep. James Courter, an aggressive conservative who is Kean's state chairman. In the audience was Washington-based, right-wing political consultant Roger Stone, who directs Kean's strategy.

What's a nice liberal Republican like Tom Kean doing with all these ferocious warriors of the right? The answer: They are tied together by a common thread -- supply-side cultural differencies that have divided the GOP since the great schism of 1912 and were widened still more by Barry Goldwater's coup in 1964.

Closing the party's gap here and elsewhere is based almost solely on tax-cut ideolgy first preached by Kemp six years ago. Kean transformed the nature of the June 2 primary to a debate on tax reduction and attacted conservative support because his program is compatible with President Reagan's. Thus, hopes for not only Republican success but party unity depend on the fate of the tax-reduction movement.

Kean is the embodiment of the liberal Republican aristocracy that provokes the party's populist right. An ancestor served in the Continental Congress, and he is the fifth generation of Keans in New Jersey. His grandfather was a U.S. senator, and his father served 20 years in the House. In a state where a majority of Republican county chairmen are Italian Americans, Kean's upper-class accent sounds more like New England than Jersey.

Kean was national youth director for William Scranton's futile campaign for the 1964 presidential nomination, and eight years later became Mr. Environment in the New Jersey legislature as the 36-year-old speaker of the Assembly. In 1978, he vainly fought for liberal Sen. Clifford Case against Jeff Bell's Republican primary challenge.

Defeated for the 1977 nomination for governor, Kean knew he must expand his base or perish. He sought out Bell for support and hired Stone, who had impressed Kean with his tenacity in organizing the state for Reagan in 1980. Together, they determined to break out of a Republican contest to be toughest against crime and for capital punishment and instead to go all out on supply-side issues.

The program was drafted by Prof. Mark Miles of Rutgers University, co-author with supply-side guru Arthu Laffer of a new economics textbook. The three-year plan would cut the state business income tax in half, eliminate the business property tax and slash the state sales tax by 20 percent. The intent: to increase tax revenue by bringing business back to New Jersey.

"A disaster!" cried Mayor Pat Kramer of Paterson, supported by the remnants of the state party's liberal leadership. Kramer, along with millionaire businessman Bo Sullivan, is Kean's principal challenger. While other Republican candidates hurridely ground out alternative tax schemes, Kramer insisted on budget reductions first. He thus plays George Bush to Kean's Ronald Reagan -- albeit a patrician, non-conservative Reagan.

It was a close enough impersonation to bring Kemp here to defend Kean's tax program. New Jersey conservative activist Harlan Schliker, supporting State Sen. James Wallwork as the authentic candidate of the right, tried to dissuade Kemp and even asked White House political aide Lyn Nofziger to keep Kemp out (Nofziger replied he wanted no part of it).

Present as Kemp praised Kean in the state assembly chambers was yet another symbol of Republican peacemaking: political consultant John Deardourff, a former Nelson Rockefeller aide who in 1978 advised his California client running for governor to oppose the Proposition 31 tax cut. In 1981, he has designed Kean's television commercials that stress tax reduction for growth.

Deadourff and Kemp have been at odds for so long that before attending the statehouse news conference, Deardourff offered to stay away. When it was over, Kemp sought out Deardourff shook hands and said: "John, its good to be on the same side." What was the origin for the rift? "Something about Kemp undercutting Rockefeller and working with Nixon back in '72," Deardourff replied. "I can't quite remember."

Ending half-remembered feuds derives from Kean's belief that Kemp and Bell, pressing for economic growth with tax rate cuts, follow the old Republican moderate tradition of optimistic faith in progress. Whether or not the tax issue nominates Kean, it has temporarily bridged real cultural, political and ideological differences between Tom Kean and Jack Kemp. The resulting common purpose is the hope of the Republican Party.