Washington — With less than a week to go to the customary Labor Day start of the fall campaign, the Republicans and Democrats are still jockeying for the starting gun advantage.
Broadly speaking, the Republicans' most powerful tactical weapon is clearly the White House. This weekend again, by vetoing a $14.1 billion appropriations bill he called a ''budget buster,'' President Reagan regained the Republican offensive. The veto, at least as far as the Washington political battle scene is concerned, has yanked the focus away from presidential backtracking on the $98.3 billion tax hike the week before.
The Democrats, still losers in parliamentary skirmishing in the nation's capital, nonetheless hold great tactical potential with other forces building for them in the political hinterlands.
''The Republicans have a lot of baggage this year,'' says John Sears, the highly regarded GOP tactician who ran President Reagan's 1980 campaign until the New Hampshire primary, ''baggage the out party can make use of.''
''First, the high rate of unemployment is of use to the Democrats,'' he says. ''The Democrats have not had jobs for an issue for a long time, since back in the '50s. This could help them turn out a lot of people in the election.
''In the '60s and after, the Democrats got off into divisive social issues. They can come into this election united. There's a lot that holds them together.
''Second, there's trouble in the farm belt, where most of the voters have been regular Republicans over the last 10 or 15 years,'' says Sears. ''In states like Indiana, Ohio, Illinois, you have a double downturn of farm and unemployment. Pockets in the South - Alabama, for instance - have high unemployment.
''Third, major groups like the elderly are worried about budget cuts. A lot of these voters haven't turned out for the Democrats recently. They may this time. And there are better opportunities for the Democrats to turn out minority voters.''
All this adds up to Democratic potential - if not yet seats in their pocket.
''The Democrats have a lot to work with,'' Sears says. ''Most of the House races haven't shaped up enough to measure. But in four Midwestern states alone there are 15 Republican districts that could go Democratic. There have to be 30 or 40 such districts through the whole country. That number could come down to the 20s by the election.''
Optimistic White House forecasts have put the probable GOP losses in the House at less than 20 seats.
While analysts strive for concrete ways to predict election outcomes, actual results usually stem from a blend of tactical maneuvers, economic trends and overseas events, plus, some experts note, an almost unconscious public impulse to keep a power balance in Washington.
The Republicans, with the Reagan White House effectively managing political debate and headlines from Washington, plus their financial and media advantages, are generally granted the tactical edge. Concerning trends and events, the Democrats, as Republican Sears acknowledges, have a number of issues going for them this year that could unite and mobilize them.
The third factor - how the public wants to calibrate the power balance in the nation's capital - modestly favors the Democrats, as it has favored the out party in every midterm election since the days of FDR.
By targeting certain states for presidential visits late in the campaign, the Republicans hope to take advantage of a Reagan campaign presence where it will help them. The White House has, for some time, planned to keep two or three days a week of Reagan's time this fall for such visits. Senate races in Virginia, Maine, Texas, California, and possibly New York are reportedly on the White House's campaign list.
The greater presidential usefulness may lie in keeping up the Republican Party's offensive against the Democrats. In contrast with last year's Reagan West Coast vacation, when stories of an idle, out-of-touch chief executive proved embarrassing to the White House staff, Reagan's team so far has kept up at least a semblance of initiative with its domestic spending veto and other actions while Reagan vacations at his mountaintop ranch.
This weekend's veto, announced in a presidential radiocast, was the first of a string of such actions anticipated by the Republican President up to the election. Such vetoes could help portray the Democrats as big spenders, and help shift the onus for disappointing economic results to the Democrats.
Surveys show this is a useful tactic. The public tends to see congressional Democrats as an obstructive force. In placing blame for delay in passing the new budget, the public by three to one singled out congressional Democrats over the President, and by nearly five to one over congressional Republicans, in a Penn & Schoen survey.