All politics may be, as the saying goes, local politics. But there definitely is a larger dimension to the battle between Republicans and Democrats for control of the United States Senate. Cast a ballot for a senatorial contestant in November, and you may help decide which party will determine Congress's domestic and foreign policy agenda for the last two years of Ronald Reagan's presidency, perhaps even which party will win the presidency in 1988.
``I think the course of the nation will depend, to a very considerable extent, on who has control of the Senate,'' says Sen. Robert H. Byrd (D) of West Virginia.
Now minority leader in the Senate, Mr. Byrd hopes to be majority leader when the 100th Congress convenes in January.
Few of this fall's congressional races are said to be linked by a national theme; local issues have dominated in almost all of them. But in Washington, talk of the 1986 elections has been dominated by musings over whether the Republicans or the Democrats will hold the Senate majority next year.
So as lawmakers scramble to put the final touches on a gaggle of bills, Senate leaders on both sides of the aisle have been carefully measuring their actions against the effects they may have on the Nov. 4 elections.
For example, during the recent Senate consideration of antidrug-abuse legislation, Sen. Paula Hawkins (R) of Florida was given the honor of managing the bill on the floor for the Republicans. This gave Mrs. Hawkins, involved in a tight race for reelection against a popular governor in a state where the war against drug abuse is a top priority, plenty of television exposure from the Senate floor.
At the same time, her Republican colleagues went to great lengths to praise Hawkins in front of the TV cameras. Republican senators are in greater danger of being knocked back into the minority than at any time since 1980, when they were swept into power on President Reagan's coattails.
Their task of defending a precarious 53-to-47 majority is complicated by the peculiar arithmetic of congressional elections: Twenty-two Republican seats are to be filled, as opposed to 12 of the Democrats' slots, so Republicans must win 56 percent of the contests if they are only to eke out a 50-50 tie that can be broken in their favor with Vice-President George Bush's vote.
History is not on the Republicans' side: The party of the president in power tends to lose congressional seats in off-year elections such as this.
If the balance of power in the Senate is tipped from the Republicans, lawmakers of both parties say, the result could have important implications for a variety of policies involving agriculture, trade, the environment, and foreign affairs, as well as judicial appointments.
That is due in part to the parliamentary prerogatives in the House of Representatives and Senate that come with being the majority party. The majority leaders set legislative schedules determining when bills come to the floor and, sometimes, under what conditions. Members of the majority party chair the committees and subcommittees through which nearly all legislation passes before being considered on the floor.
The power of the majority is stronger in the House than in the Senate, where individual members are given far more liberty to affect the course of debate on a bill. But the Senate majority's power is considerable.
So in his final two years as President, Mr. Reagan would likely face a more independent, even defiant, Congress if Democrats controlled both Senate and House.
Judicary nominees would be subject to more intensive scrutiny by a Democratic Senate, perhaps thwarting the administration's stated goal of stocking the nation's federal benches with conservatives. Indeed, a number of lawmakers believes that the confirmation hearings of Chief Justice William H. Rehnquist would not yet be completed if the Democrats were running things.
Also, the looming prospects of a race for an open presidential race would stiffen the resolve of Democratic leaders to oppose Reagan, according to one school of thought, as Democrats seek to distinguish themselves from the White House and Republican presidential candidates.
``I don't think there's any doubt about it that the President would be hampered. Just look how he fares in the House,'' says Senate majority leader Robert Dole (R) of Kansas. ``If the Democrats take control ... they'll slow down the possibility of compromise with the President on all kinds of issues.''
Such an occurrence could also sharply affect a few presidential prospects. Senator Dole, for example, has effectively used his position as majority leader to build national visibility and lay the groundwork for a presidential bid in 1988. His prominence might diminish as minority leader.
Similarly, the visibility of such stalwart Republican Senate committee chairmen as Agriculture's Jesse Helms of North Carolina and Richard Lugar of Indiana, chairman of the Foreign Affairs Committee would decrease as they surrendered their chairmanships.
At the same time, the prospects of such potential Democratic presidential hopefuls as Sen. Joseph Biden of Delaware and Sen. Sam Nunn of Georgia could improve in a Democratic Senate.
It is expected that Mr. Biden would inherit the chairmanship of the Senate Judiciary Committee and Mr. Nunn would inherit the chairmanship of the Senate Armed Services Committee.
Some Republicans put an optimistic twist on the possibility of a Democratic majority in the new Senate, saying it could ensure a GOP presidential victory.
``If the Democrats take over, it's a Democratic Congress, and whatever happens in '87 and '88 is going to be on their doorstep,'' says Senator Dole.
``So we'll be ready to roar back in power in '88.''