Sizing up Senate races for '82.

The Republican drive for US Senate mastery will likely reach its high-water mark in next fall's elections, experts here agree. Some historic names in Senate annals - Henry M. Jackson of Washington and minority leader Robert C. Byrd of West Virginia - could face stiff challenges. The outcome could cut into the number of seats held by the Democrats, reliable GOP strategists contend.

Only Sen. Edward M. Kennedy of Massachusetts and William Proxmire of Wisconsin are rated unchallengeably strong among the 21 Democrats up for reelection in 1982, concede some Democratic sources. The GOP have fewer senators up for reelection - only 12 - and fewer candidates whose reelection is in serious doubt - S. I. Hayakawa in California and Lowell P. Weicker Jr. in Connecticut.

As of now, a Monitor survey of the 33 Senate races next year shows a likely gain of three to five Senate seats for the GOP, hiking their Senate total to 56 or 58 seats. But in 1984 and 1986, the GOP will parade a number of relatively weak incumbents, and power is likely to begin to tilt back to the Democrats.

The races as of now will not hang very much on national issues. ''National politics is a wash at the moment,'' says Michael Barone, a political analyst.

''GOP gains would not be due to any conservative trend,'' says Thomas Mann, a congressional affairs expert.

Another easy prediction is that some of the races will be donnybrooks. When Ohio Democrat Howard Metzenbaum faces off against longtime rival John Rhodes, now the state's governor, a ''street fight'' is anticipated by political observers there. Senator Metzenbaum at the moment is given the edge, and Mr. Rhodes must first defeat Rep. John Ashbrook in a primary.

Other races are, at the least, complicated. Already, nine major candidates are running for Senator Hayakawa's California seat. If the incumbent by some chance survives a primary - with the President's daughter Maureen splitting the vote with San Diego Mayor Pete Wilson and Congressmen Barry Goldwater, Jr. and Pete McCloskey - the GOP would likely lose the seat. But if Mr. Wilson emerges with the GOP nomination, he could possibly defeat Democratic Gov. Edmund G. Brown Jr.

By region, here is how the 1982 races look:

In the East, Maine Democratic Sen. George Mitchell's seat looks as if it will go to the GOP Rep. David Emery. Maryland Democrat Paul Sarbanes will hold his seat, say the Democrats. But the GOP now rates Mr. Sarbanes a loser.

New Jersey Democrat Harrison Williams may soon have his seat taken away by his Senate colleagues for his Abscam role. If the GOP pits feisty US Rep. Millicent Fenwick against a Democratic appointee like James J. Florio, the Republicans could win, both sides agree.

And in New York, Democrat Daniel P. Moynihan is given the edge to survive by the Democrats, but Republicans see the race as nearer even with the right candidate opposing him, such as Rep. Jack Kemp.

Apart from Connecticut's Weicker, who could defect from the GOP to run as an independent or Democrat, the Republican Eastern incumbents - William V. Roth in Delaware, John Heinz III in Pennsylvania, John H. Chafee in Rhode Island, and Robert T. Stafford in Vermont - seem secure.

In the Midwest, Democrat Donald Riegle in Michigan is rated a tossup by both parties, with the prospect of Republican Gov. William Milliken entering the Senate race. Democrat Edward Zorinsky in Nebraska is rated fairly safe, while Democrat Quentin N. Burdick of North Dakota is given about an even chance. Republicans in the Midwest count Indians's Richard G. Lugar a winner in 1982. Incumbent Republican David Durenberger of Minnesota is given the edge by both parties. And Missouri's John C. Danforth looks solid - unless Democratic Rep. Richard Gephardt, facing a redistricting challenge, takes him on.

In the West, the Republicans are given the edge to keep the seats of Harrison H. Schmitt in New Mexico, Orrin G. Hatch in Utah, and Malcolm Wallop of Wyoming.

Strategists for both parties see Democrats Dennis DeConcini of Arizona, Spark M. Matsunaga of Hawaii, and John Melcher of Montana keeping their seats. The GOP thinks it has a chance for Mr. Matsunaga's seat. And the Republicans expect the Democrats to yield Howard W. Cannon's seat in Nevada. The Democrats give themselves an edge in Nevada, and a clear edge for Jackson in Washington.

In the South, both sides see a likely Democratic loser in whoever is nominated for John C. Stennis's Mississippi seat, assuming he retires. Both sides agree Democrat Lawton Chiles is fairly safe in Florida. But the Republicans think they can beat James R. Sasser in Tennessee. And they rate their prospects for LLoyd Bentsen's Texas seat, Harry F. Byrd Jr.'s in Virginia, and Robert C. Byrd's in West Virginia, as even. Democrats see the Harry Byrd race in Virginia as even and an edge in Texas, West Virginia, and Tennessee.