Washington — With the momentum of youth on their side, the Republicans say they will widen their majority in the Senate by three-to-nine seats in 1982. They see the big Democratic Senate class of 1958 --when the GOP lost a whopping 15 freshman seat near the end of President Eisenhower's second term -- continuing to fall by the wayside in the contest that comes at the halfway point of Ronald Reagan's first term.
"The really interesting story is the younger people moving into the Republican Party," says Thomas Mann, congressional expert for the American Enterprise Institute and incoming president of the American Political Science Association. "In both houses of Congress, the REpublicans are the party of youth."
Mr. Mann sees a net shift of at least three seats to the GOP in the Senate in 1982.
Mr. Reagan's presence on the ballot helped the Republicans gain Senate control in the election just past. But the White House needs only to show a positive change in the direction of the nation's economy, not giant strides, to help again, says Sen. Robert Packwood, chairman of the National Republican Senatorial Committee.
Reagan, however, is far from the GOP's only Senate resource for 1982, Senator Packwood says. More Democrats are up for reelection than Republicans (20 Democrats to the GOT's 12). And more Democrats, proportionately, are vulnerable.
Even such Democratic notables as Sen. Daniel P. Moynihan (D) of New York now look beatable. Surveys show former Rep. Elizabeth holtzman ahead of Mr. Moynihan 43 percent to 38 percent in a primary matchup, says Robert Teeter, pollster for the National Republican Committee. Only 34 percent of New Yorkers say they want Mr. Moynihan reelected.
"We start out ahead organizationally," Senator Packwood says. The GOP Senate reelection committee announced it will spend at least $8.5 million on the 1982 contests. Packwood said it will take time for the Democrats to find the 200,000 or 300,000 donors to give that kind of money.
But he -- and some neutral observers -- feel the momentum of youth has now switched to the Republican side and will be even more decisive in 1982 than these technical advantages.
"Our class of 1980 is our closest equivalent to the Democratic class of '58," Packwood says. In 1958, the Democrats leaped from a 49-to-47 seat edge to a 64 -to-34 margin over the Republicans that helped steamroller Democratic legislation through the Congress for the next 20 years.
He says that 1982 "will be the key" to a Republican era."For at least one more election we'll have a lot of 35-year-old state legislators opposing 70 -year-old Democrats."
The GOP may get some Senate help from popular governors like Michigan Gov. William Milliken going after Senate seats.
Here's how the 1982 reelection prospects of senators look as of now, by the reckoning of Thomas Mann and other observers:
* Strong: Democrats Robert Byrd of West Virginia, Lawton Chiles of Florida, Henry Jackson of Washington, Edward Kennedy of Massachusetts, Spark Matsunaga of Hawaii, and William Proxmire of Wisconsin; Republicans Richard Lugar of Indiana, William Roth Jr. of Delaware, and Malcolm Wallop of Wyoming.
* Less strong but with an edge: Democrats Dennis DeConcini of Arizona, John Melcher of Montana, Daniel Moynihan of New York, and Paul Sarbanes of Maryland; Republicans John Chafee of Rhode Island, John Danforth of Missouri, David Durenberger of Minnesota, Orrin Hatch of Utah, John Heinz of Pennsylvania, and Harrison Schmitt of New Mexico.
* Given an even chance at best: Democrats Howard Metzenbaum of Ohio, Donald Riegle of Michigan, and James Sasser of Tennessee; Republicans S. I. Hayakawa of California and Lowell Weicker of Connecticut.
* In clear trouble: Democrats George Mitchell of Maine and Harrison Williams of New Jersey.
The Democrats have at least five senators who could retire, led by John Stennis of Mississippi, always a sure winner in the past. The others include Quentin Burdick of North Dakota, Howard Cannon of Nevada, Henry Jackson of Washington, and Paul Sarbanes of Maryland. Nebraska's Edward Zorinsky may shift to the GOP in 1982.
Connecticut's Republican Weicker may retire or switch parties, Mann says, predicting "his erratic behavior will catch up with him."
Mann sees the Democrats losing fiv e seats, the Republicans two, for a net GOP gain of three.