WASHINGTON -- Americans started voting across the U.S. on Tuesday, with Republicans having clear prospects of gaining control of the Senate and almost as good a chance of taking over the House of Representatives for the first time in the lifetime of most Americans.


Assuming that toss-up races divide evenly between the parties, Republicans would have a 51-49 advantage in the new Senate -- a gain of seven seats and a majority for the first time in eight years, according to a final 50-state survey by The Washington Post.


A similar allocation of House seats too close to call would give the Republicans 214 -- four short of a majority but a 36-seat gain.


Although not on the ballot, President Bill Clinton has been a major focus of the campaign because of his low approval rating. Such Republican gains are likely to make it far more difficult to get legislation through Congress during the second two years of Clinton's term.


Late interviews with party officials, pollsters, campaign consultants and neutral observers in all 50 states suggest clearly that the voters' impatience with incumbents and unhappiness with Clinton may tilt the close races -- like Virginia's contest between Senator Charles S. Robb (D) and Iran-Contra figure Oliver L. North -- toward the Republicans. If they win three-fourths of the toss-ups, the Republican majority in the Senate might swell to 54-46, and Republicans might gain a 225-210 majority in the House.


Republicans have not controlled the House in 40 years.


A major caveat: With many of the closest contests in states and districts where no incumbent is on the ballot, and with some late polls still showing one-fifth of the voters hedging their choices, forecasts may be worth even less than usual this year.


Winning the majority of the close races would permit the Democrats to maintain nominal control of the 104th Congress. But that Congress is clearly going to be more conservative than the last one.


The 1994 campaign has broken the record for off-year political spending, and much of that money has gone for television spots. The negative character of most of the ads is expected by many observers to depress turnout, except in a few states like Tennessee that have exceptionally hot races.


The difference in enthusiasm between the parties is most pronounced across the South and in the Rockies and Southwest, where anti-Clinton sentiment is strongest.


In addition, there is clear evidence that nominally nonpartisan groups -- especially the Christian Coalition and the National Rifle Association -- are likely to be a more powerful force for the Republicans in this election than ever before.


New York's Democratic governor, Mario M. Cuomo, may be able to join Senator Edward Kennedy, Democrat of Massachusetts, on the list of durable liberals who defy the election odds. Kennedy looks safe now, after an earlier scare, but Cuomo is still sweating out his race.


But other big-name Democrats have worse problems.


Representative Dan Rostenkowski, Democrat of Illinois, is facing the end of his congressional career. Chicago Democratic voters are telling their leaders that the veteran, indicted for misuse of government funds, has to go, even though they know virtually nothing about his opponent, 31-year-old Michael Patrick Flanagan.


Jack Brook, the Texas Democrat who is chairman of the House Judiciary Committee and one of the legendary strongmen of Capitol Hill, is fighting for his political life in his Beaumont district. Representative Sam Gibbons, Democrat of Florida, who took over as acting chairman of the House Ways and Means Committee after Rostenkowski's indictment, has had a tough battle in Tampa.


Senate Budget Committee Chairman Jim Sasser, a strong prospect to succeed retiring Majority Leader George Mitchell as head of the Senate Democrats, has had difficulty translating his Washington influence into votes in Tennessee. He has no more than a 50-50 chance of heading off his Republican opponent, Bill Frist.


Many of Clinton's closest political allies -- people like Governor Zell Miller of Georgia, who nominated him at the 1992 Democratic Convention, and Representative Dave McCurdy of Oklahoma, who made a seconding speech -- are in very tough battles.


But election night may be a nail-biter for former President George Bush, as well. Two of his sons are in hard-fought gubernatorial battles. Contrary to earlier estimates, George W. Bush may have a bit better chance of defeating Texas Governor Ann Richards, a Democrat, than Jeb Bush has against Florida's Democratic governor, Lawton Chiles.


Overall, governors' races have not developed as well as Republicans had hoped earlier in the year, when they were predicting that a majority of all states would have Republican governors.


In California, Republican Governor Pete Wilson's comeback against State Treasurer Kathleen Brown looks solid enough to last through Election Day. But Republican State Senator George Pataki, is now the underdog against Cuomo in New York.


As for the Senate, the consensus is that Republicans, who now have 44 seats, will almost certainly take over from Democrats in Arizona, Maine, Ohio, and the Tennessee seat once held by Gore.


Democratic Senator Dianne Feinstein is involved in a close fight against Republican Representative Michael Huffington, who has spent $28 million of his personal fortune in the most expensive Senate campaign in history.


The Republicans also have good to excellent prospects in Michigan, Oklahoma, Virginia, Pennsylvania, and against Sasser in the second Tennessee race. The best chance for Democrats to win a Republican-held seat is the open seat in Minnesota, with lesser possibilities in Delaware, Vermont and Washington. If the Republican tide is very strong, Democrats could also be in trouble in New Jersey and New Mexico.


In the House races, multiple-seat gains are possible for the Republicans in Arizona, California, Florida, Georgia, Illinois, Indiana, Michigan, New Jersey, Ohio, Oklahoma, Oregon, Tennessee, Texas and Washington.