Contrary to expectations, it looks now as if continued Republican control of the U.S. Senate will depend directly on President Reagan's ability to interject himself into the key races in the November election. Democratic prospects in the one-on-one matchups have improved enough during the spring primary season that GOP strategists now concede the president may be their best bet for holding their shaky Senate majority.

"The hole card for most Republicans in marginal states will be The Man," said Sen. Paul Laxalt, the general chairman of the Repubican Party. "His appearance late in October will be worth at least a point or two."

A look at developments in a dozen states shows you why Republicans will need Reagan's ''coattails,'' even though he isn't on the ballot, if they are to stave off the four-seat swing that would put the Democrats back on top.

In Louisiana, Democratic Rep. John B. Breaux has finally made peace with organized labor and is starting to sneak up on Republican Rep. W. Henson Moore in the battle to succeed retiring Sen. Russell B. Long.

This is a must-win state for the Republicans, to offset the expected Democratic capture of the Maryland seat of retiring Sen. Charles McC. Mathias. A loss by front-runner Moore in the early Sept. 27 voting would send shock waves through the GOP and shift thousands of nervous political-action committee (PAC) dollars to other Democrats in anticipation of a switch of Senate control.

Sen. Paula Hawkins (R) still trails Florida Gov. Bob Graham (D), and her fund-raising for the expensive race was stalled by her prolonged hospitalization for back surgery this spring.

Meantime, the Democratic threat has grown in three other southern and border states with GOP seats. In Alabama, Rep. Richard C. Shelby won the Democratic nomination without a runoff and is within striking distance of Sen. Jeremiah Denton. Ex-Gov. Terry Sanford did the same thing in North Carolina and has, at least temporarily, the upper hand over Rep. James T. Broyhill (R) for the seat of retiring Sen. John P. East. In Oklahoma, Democratic Rep. James R. Jones has put the skeptics to rout by announcing he will commit his million-dollar war chest to challenging Sen. Don Nickles.

The biggest jump in Democratic prospects has occurred in Pennsylvania. Primary winner Rep. Bob Edgar looms as a serious threat to Sen. Arlen Specter, who wasn't even on the GOP's ''worry list'' two months ago. Edgar, who represents a normally Republican district outside Philadelphia, could undercut Specter's southeastern Pennsylvania base.

Despite his victory over Gov. Bill Janklow (R) in last week's South Dakota primary, Sen. James Abdnor (R) remains an underdog against Rep. Thomas A. Daschle (D). In Idaho, Sen. Steve Symms (R) is in a close battle with retiring Gov. John V. Evans (D). And to add to the GOP's western worries, in Nevada, where Laxalt is retiring, ex-Rep. Jim Santini, the Republicans' party-switching candidate, is off to a very rocky start against Rep. Harry Reid (D).

There has been some offsetting good news for Republicans in four states. In California, their best bet for November, Rep. Ed Zschau, threaded his way through a crowded primary field to gain the nomination against Sen. Alan Cranston (D). And in Colorado, Rep. Ken Kramer has avoided a primary and can concentrate on catching up to Rep. Tim Wirth (D) for the seat Sen. Gary Hart (D) is vacating.

Ex-Gov. Christopher (Kit) Bond of Missouri continues to keep pace with Lt. Gov. Harriet Woods (D) for the seat of retiring Sen. Thomas F. Eagleton (D), and in Vermont, ex-Gov. Richard Snelling (R) has his campaign in gear to try to overtake favored Sen. Patrick J. Leahy (D).

Calculating only on the basis of individual matchups, the Republicans are clearly in trouble -- and that does not count such states as Georgia, North Dakota, Washington and Wisconsin, where Democrats have at least long-shot possibilities.

But the general climate of political complacency -- marked by very low turnouts in most primaries -- should favor incumbents, and Republicans have 18 incumbents in Senate races compared with the Democrats' nine.

The other possible equalizer is Reagan. Democrats discount him as a factor, noting that he had few coattails even in his 1984 landslide. But he has raised millions for the Republican candidates, and they count on him for help again in November. He can jack up Republican turnout, with recorded phone messages to GOP voters and late personal appearances. And he will try to make wavering voters understand that with control of the Senate at stake, a vote for the Democrat in their state is really a vote against him.

In North Carolina, Alabama, Idaho, Nevada and California -- and even in such troubled farm or energy centers as Louisiana, Oklahoma, Colorado, North and South Dakota -- Reagan's popularity is so high that his personal campaigning could make a difference.

Whether he can swing those states or not, pressures are building for him to try -- so 1986 becomes much more of a personal referendum on Reagan than it first appeared.