Electioneering does not begin in earnest until after Labor Day. But in the wake of a successful Democratic convention, President Reagan is picking up the gauntlet thrown down by a strengthened, more buoyant Democratic Party.
Quickly taking the political offensive, Mr. Reagan sets out this week for a two-day political trip to Texas, Georgia, and New Jersey. Vice-President George Bush will also make a swing through Texas, linking up with Reagan for a rally in Austin on Wednesday.
All three states are important. Texas, with its 29 electoral votes, is crucial to the President's reelection and is expected to be a toughly fought-over battleground.
''When it came time to get to the Democratic convention and (Sen. Lloyd M.) Bentsen was unceremoniously dumped by Walter Mondale, we thought it was time to highlight that we do have a Texan (Mr. Bush) on the ticket,'' a Reagan-Bush campaign committee official says.
In Georgia, the only Southern state that he did not win in 1980, Reagan is expected to spotlight the Mondale connection with former President Jimmy Carter and firm up Reagan's support in the South in general. And in New Jersey, he can demonstrate that he is not afraid to go after the votes of urban ethnic and Roman Catholic constituencies to which the vice-presidential candidacy of Geraldine A. Ferraro is expected to appeal.
''Reagan is seizing on an opportunity and taking the offensive at the earliest point,'' says William Greener, communications director of the Republican National Committee. ''We were surprised at the number of openings left to the Republican Party as a result of the convention - including writing off the South and Texas.''
As the President prepares to test some campaign themes on the hustings, a new Gallup poll shows Mr. Mondale moving ahead of the President for the first time. In a telephone survey of some 1,000 registered voters, Gallup found that 48 percent supported a Mondale-Ferraro ticket and 46 percent the Republican ticket. Published in Newsweek magazine, the poll has a four-point margin of error, which means that Reagan could actually be trailing or leading his Democratic rival.
Mondale and fellow Democrats are encouraged by the poll results. But political analysts urge caution.
''It doesn't mean much at this stage,'' says Norman J. Ornstein, a political scientist at the American Enterprise Institute. ''The Democrats simply did not want slippage from the convention but to stay within striking distance of Reagan. But after the Republican convention at Dallas, the President will pull ahead - and only after that convention will people begin to focus on the alternatives.''
''We've said all along that it's inevitable the race would tighten up, and so we shouldn't be surprised or get excited,'' Mr. Greener says.
GOP strategists remain confident that Reagan's continuing popularity and the upbeat economic picture will be the most potent factors in the coming election. But as they also forecast would happen, the Democrats emerged from their convention in much better shape, after a fractious primary battle.
Hence, the repeated warnings to Republican troops not to become complacent. ''They have to get in shape now for the election, like spring training,'' says presidential scholar Thomas Cronin of Colorado College. ''Not because they're scared of the Democrats, but whoever holds office tends to know the possibilities of things going wrong.''
While the Reaganites are not gripped by worry, analysts say, they must note the gains from San Francisco: The convention turned out to be more unified than most thought possible. It was well run. Mondale, despite the Bert Lance affair, came out swinging, delivered a good speech, and took control. Ms. Ferraro, if a political gamble at this point, lends zest to the ticket. And, to GOP discomfort , the Democrats are trying hard to recover the Reagan themes of family values, hard work, and patriotism.
''Mondale at this point is talking about conservative issues and trying to overcome the fact that he's an old-time liberal,'' says GOP campaign operative. ''It remains to be seen whether the American people see through that. It's hard to reconcile the Democrats being a party of traditional values with a party that had a gay caucus represented at its convention.''
Most political observers say the Democrats have a stiff battle ahead. But they no longer seem to rule out a tight race. If Mondale can stay close, chipping away at Reagan on issues of fairness, the deficit, and peace, and gear his campaign to family values and tradition, he may begin to raise doubts in people's minds about Reagan policies, experts say. Then, if he can best Reagan in two or three debates, he might have a shot at the presidency.