While modestly impressive Republican gains Tuesday pose a threat to President Carter in 1980, they also guarantee two years of intensified Republican civil war leading to the presidential nomination.

The party's conquest of four major governor's chairs (most spectacularly is Texas) undermines the presumption of Ronald Reagan as the party's nominee. Beyond personalities, it sets up this debate: Does the Republican path out of the wilderness lie in the tax revolt or in stressing a new, well-scrubbed image?

Ammunition for both sides is being drawn from this year's Republican victories by leading proponents of the two tactical schools: Rep Jack Kemp of New York and the Washington-based campaign consulting firm of Bailey and Deardourff.

In a possible preview of 1980, Kemp clashed with John Deardourff in Perry Duryea's losing race against Gove. Hugh Carey of New York. The author of the Kemp-Roth tax-reduction bill believes Duryea failed because Deardourff stressed image while campaign mastermind David Garth stressed issues for Carey.

The Bailey-Deardourff firm cannot be blamed for Duryea's maladroit performance, particularly his refusal to heed their pleas to publish his tax returns. Bailey-Deardourff, making no secret of their contempt for Kemp-Roth tax reduction as a campaign device, were involved in highly successful media-oriented campaigns in Pennsylvania, Michigan, Ohio, Illinois and Tennessee.

Their formula was at its best in Pennsylvania where a brilliantly crafted media campaign brought former U.S. Assistant Attorney General Richard Thornburgh from oblivion to snatch the governorship form ex-Pittsburgh Mayor Peter Flaherty. The formula Stress Thornburgh's qualities, undermine Flaherty's credibility and play down iddeology and issues.

Thornburgh is the newest and one of the most impressive of a group of moderate Republican governors in a central and politically centrist belt running from Pennsylvania to Iowa, whose success last Tuesday endangers the inevitability of Reagan. There has been private discussion of those governors pooling their power to pick a 1980 candidate - a process complicated by the presidential ambitions of Illinois' James Thompson and possibly Iowa's Robert Ray.

But also joining the Republican governors is a different breed: multi-millionaire Dallas industrailist William Clements, who surprised even his own advisers by being elected governor of Texas. Clements, tough-talking and tough-looking, is the antithesis of the image candidate. His lavish media campaign stressed pro-taxcut, anti-government, anti-liberal ideology.

Envisioning Bill Clements at a Republican governors' conference delights his friends. "Imagine hime with those nice candy-coated fellows like Ray and Milliken [Gov. William Milliken of Michigan]," one insider told us. "Clements will roll'em over."

But not in Reagan's behalf. Clements a Ford man in 1976, felt Reagan was not helpful in the early stages of his race for governor. He still burns over the intervention by the Reaganite Citizens for the Republic in a Republican congressional primary in Texas against Clements' advice. Texas state Republican chairman Ray Barnhart, a faithful and ardent Reaganite, now is eclipsed by a Republican governor who obviously would prefer George Bush over Reagan for president.

Ironically for Reagan, Tuesday's ideological tone was slightly rightward, with the mass slaughter of all incumbent liberal Democratic senators (especially Sen. Dick Clark of Iowa) who were supported by the National Committee for an Effective Congress. What's more, the president's own advisers concede that victories of the three Republican senators - Jesse Helms, Strom Thurmond and John Tower - stigmatized as "radical rightists" confirms the futility of unadorned anti-conservative assaults.

Furthermore, Kemp insists on the validity of Kemp-Roth, which is derogated by both the White House and Bailey-Deardourff. Rep. Bill Armstrong's Senate triumph in Colorado and Newton Gingrich's for the House from Georgia were squarely based on unswerving advocacy of massive tax cuts.

Kemp must soon decide whether this merits his own presidential bid, long urged by close advisers. But whether he, Reagan or anybody else opposes Carter with a national tax-reduction cursade, Tuesday's results ensure an internal Republican struggle for 1980 that seemed most improbable a few days ago.