THE MORE WE rummage through Tuesday's returns, the more we think it's shortsighted to be-little the Republicans' advances. True, the GOP's net gains in Congress were modest - three Senate seats and a dozen in the House. But the statehouse story is something else. The party netted six governorships, boosting its total from 12 to 18. It gained majorities or ties in 15 state legislative houses, while losing none, and overall picked up about 300 state-legislative seats - after losing about 800 since 1972.

To put this rebound in perspective, before Tuesday the Democrats enjoyed total control in 30 states. The GOP broke 10 of those monopolies, including those in Minnesota, Texas, Pennsylvania, Wisconsin and Tennessee, while letting only one state, South Carolina, slip completely away.

Besides giving Republicans more power and patronage right now, these victories bolster the GOP's position for the crucial 1980 elections and the congressional redistricting and state-house reapportioning of 1981 and 1982. This was what Republican national chairman Bill Brock had foremost in mind when he gave top priority to grass-roots rebuilding and invested about $2 million in state legislative campaigns. Mr. Brock has figured that redistricting by Democratic legislatures after the 1970 census cost the GOP about 40 House seats, plus dozens at the state level. Another round like that could doom the party to minority status until 1990 or beyond. Now, barring setbacks in 1980, the Republicans can at least protect themselves in more states and control some, such as Indiana, where redistricting is ferociously partisan.

Those 300 new Republican legislators also mean major gains in new talent and grass-roots organization, assets which the party sorely needs. State legislatures are the traditional training-grounds for higher office; according to one count, 32 of the 77 House newcomers elected this week have had state legislative experience. Local-level recruiting failures of the past, plus the erosion of GOP organizations, were big factors in the party's inability to field strong candidates for many House seats this year. In this sense, too, Mr. Brock is right: a Republican rejuvenation has to start - and may indeed be starting - from the bottom up.