Republicans are planning a vigorous pursuit of state lawmaking seats and hoping for at least modest gains. It is all part of a long-range drive to give the party ''control of a majority of the state legislatures by 1991,'' the next time for redistricting, explains national GOP chairman Frank J. Fahrenkopf Jr.
Noting that his party currently controls the legislatures in only 11 of the 50 states, he says party rebuilding efforts at the state level were given a big boost last year, when a special election in Michigan shifted the state Senate from Democratic to Republican hands.
''I think we should pick up two, three, or four seats at least in many states this November,'' he asserted while in Boston for meetings at the annual National Conference of State Legislatures.
Mr. Fahrenkopf made it clear that major emphasis in party building will be on fielding strong contenders for legislatures and local offices.
''Reelecting President Reagan and retaining control of the (US) Senate is only part of the job that needs to be done,'' Fahrenkopf said, urging GOP legislators to return home and work for a big victory in November.
''We set out to register 2 million new Republican voters during the current (two year) election cycle, and to date we have 1,250,000 signed up,'' he says enthusiastically.
Particular attention is being directed to registering potential GOP voters among American citizens abroad, he says. Toward that end, the Reagan-Bush campaign has people in 52 nations helping make sure Americans there are not only registered but encouraged to cast absentee ballots.
Fahrenkopf notes that in the 1982 California governor's race, it was ''absentee ballots which turned things around and elected (Republican) George Deukmejian governor.''
While hardly elated over last week's Gallup poll showing the Reagan-Bush ticket trailing the Democratic ticket of Walter Mondale and Geraldine Ferraro by two percentage points, Fahrenkopf says he is ''not all that surprised.'' He says a surge of support for the Democratic ticket could be expected right after the convention, adding that he expects a similar movement for the Republican ticket after next month's GOP national convention in Dallas.
''One of the greatest obstacles'' faced by his party in the presidential campaign this year is ''overconfidence,'' Fahrenkopf cautions.
While stopping short of forecasting continued Republican control of the US Senate, he says ''things are looking better.''
He concedes that certain senatorial seats now held by members of his party could be in jeopardy this fall, but he also suggests that at least a couple of the 14 Democratic Senate seats at stake might be well within GOP reach.
As Fahrenhopf views it, ''there are two key issues in the presidential campaign: a strong economy and leadership.'' In both, he contends, President Reagan has proved himself.
Fahrenkopf says he thinks a single debate between the presidential candidates would be adequate, suggesting that more than a couple ''might be counterproductive,'' since the public already is pretty familiar with the positions of the two candidates.
As for a debate between Republican Vice-President George Bush and Democratic vice-presidential nominee Ferraro, the GOP national chairman says he is ''not sure it would add all that much to the campaign.''