The battle for control of the Senate after 1986 is taking on a new look, with go or no-go decisions by candidates in a half-dozen states changing the shape of the field, though not necessarily altering the odds for continued Republican dominance.

During the past few days, Republicans have picked up a strong potential candidate for their new vacancy in Nevada but lost a well-regarded backup in North Carolina. They confront increasing prospects for bruising battles between their governors and incumbent senators in Pennsylvania and South Dakota.

Democrats face the growing likelihood that Sen. Gary Hart (D-Colo.) will bypass a reelection bid in 1986 to run for president again in 1988, making that an open seat. They have House members ready to jump into Senate races in Georgia, Nevada, Oklahoma and Colorado but face likely turndowns in North Dakota and Wisconsin.

Overall, neither party has gained a clear advantage from the shuffling. Democrats, who now hold 47 seats, must make a net gain of four to dump the Republican Senate majority, but will be defending only 12 seats compared with the GOP's 22.

Hart, the 1980 Democratic presidential nomination runner-up, may postpone his announcement from November until January, his administrative assistant, Bill Dixon, said yesterday. But there is growing evidence that he is prepared to hand off the ball in Colorado to Rep. Timothy E. Wirth (D-Colo.).

"Gary has encouraged Tim to expand his base in Colorado and raise money," Dixon said. Wirth is doing just that, skipping the resumption of Congress to make a five-day trip to Israel with potential supporters.

Robert Beckel, a Democratic consultant who recently brought Wirth's administrative assistant, David Aylward, into his firm, said, "I can't conceive of any way that Hart is going to run for the Senate . . . . I think Wirth will be the nominee." At least four Republicans, including Rep. Ken Kramer (R-Colo.) and Republican state Chairman Howard (Bo) Callaway, are interested in the nomination.

Another Democrat who burst onto the national scene in 1984 -- vice presidential nominee Geraldine A. Ferraro -- is also delaying until at least November a decision about challenging freshman Sen. Alfonse M. D'Amato (R-N.Y.).

"I would love to be in the Senate and love to see Al D'Amato not in the Senate," said Ferraro, who has formed an exploratory committee. But she hasn't raised funds, and some Democrats read her delay as a signal she is backing away from a race that is certain to dredge up the allegations about her family finances that haunted her last year.

Whatever Ferraro does, Brooklyn District Attorney and former representative Elizabeth Holtzman is expected to seek the Democratic nomination.

The worst August surprise for the Republicans was the retirement announcement of Sen. Paul Laxalt (R-Nev.), but the pain was eased considerably when ex-representative James R. Santini announced Friday that he was switching his party registration from Democratic to Republican.

Santini, who served as congressman at-large from 1977 to 1982, when he lost a close race for the Democratic senatorial nomination, said that for personal reasons, "at this point, I cannot make a commitment to a Senate race in 1986." But some Republicans hope he can be persuaded to change his mind. Rep. Harry M. Reid (D-Nev.) said he is "certainly leaning toward" seeking the Democratic nomination. On the Republican side, Rep. Barbara F. Vucanovich (R-Nev.) and Attorney General Brian McKay are reportedly considering running.

In North Carolina, where the health problems of Sen. John P. East (R-N.C.) cloud his reelection bid, Rep. James T. Broyhill (R-N.C.), a potentially strong backup candidate, said last week he will run for reelection in 1986. Broyhill faced the possibility of competition in a Senate primary from a more conservative candidate backed by the Congressional Club, Sen. Jesse A. Helms' (R-N.C.) home-state political organization.

On the Democratic side in North Carolina, ex-governor James B. Hunt Jr., who lost a bitter and expensive challenge of Helms in 1984, is reported to be very close to a decision on trying again for the Senate. According to close associates, Hunt has been discouraged from running by some former supporters who say wounds from the fight with Helms need more time to heal. But with Broyhill out, Hunt is said to think his chances are improved.

If Hunt steps aside, the interested Democrats would include ex-governor Terry Sanford, recently retired as president of Duke University, William Friday, president of the University of North Carolina system, and Rep. Charlie Rose (D-N.C.). Possible Republican contenders, if East retires, are Congressional Club Chairman Tom Ellis and Campbell University President Norman Wiggins.

Another incumbent whose status worries the GOP is Sen. Charles McC. Mathias Jr. (R-Md.). Tom Griscom, director of the Republican Sentorial Campaign Committee, said Mathias' decision "is very much up in the air."

With Gov. Harry R. Hughes (D) and Rep. Barbara A. Mikulski (D-Md.) eyeing the senatorial race, Republicans do not have an established backup candidate if Mathias steps down.

In Pennsylvania and South Dakota, by contrast, the GOP may have freshman incumbents facing strong challenges from retiring Republican governors. Pennsylvania Gov. Richard L. Thornburgh (R), who beat Sen. Arlen Specter (R-Pa.) for the gubernatorial nomination in 1978, is showing increasing interest in challenging Specter.

The senator has been under fire from conservative Republicans for several votes against the Reagan administration, and they urged Thornburgh to get into the race. Thornburgh's executive assistant, Jay Waldman, said Thornburgh "was kind of cruising along, not really taking this stuff all that seriously, and then he starts hearing from all sorts of heavyweights in the party. They're telling him that Specter can't win, that he wouldn't be dividing the party with a primary challenge, he'd be saving the seat."

As a result, Thornburgh has commissioned a survey from presidential pollster Richard Wirthlin, with the results due late this month and a decision expected soon after. A poll last month showed Thornburgh leading Specter by 13 points.

Rep. Bob Edgar (D-Pa.) and Auditor General Don Bailey (D) lead the current Democratic field.

In South Dakota, private polls show Gov. William J. Janklow (R) leading Sen. James Abdnor (R-S.D.). The mayor of Watertown told reporters that Janklow had told him he will run. The governor denied this, but an announcement of his candidacy is expected. Rep. Thomas A. Daschle (D-S.D.) is expected to be the Democratic nominee.

In other states, recent developments include:

*Georgia: Rep. Wyche Fowler Jr. (D-Ga.), though still unnannounced, is regarded as the likeliest challenger to Sen. Mack Mattingly (R-Ga.).

*North Dakota: Rep. Bryan L. Dorgan (D-N.D.), though leading Sen. Mark Andrews (R-N.D.) by 25 points in a private poll, appears inclined to wait for 1988 and the expected retirement of Sen. Quentin N. Burdick (D-N.D.). Dorgan said, "The pressure to run in 1986 has been heavy, but I'm still trying to decide if I can make the personal commitment a campaign would take."

*Oklahoma: With Gov. George Nigh (D) taking himself out of the picture, Rep. James R. Jones (D-Okla.) has become the favorite for the Democratic nomination against Sen. Don Nickles (R-Okla.). Jones said he will wait until after tax bill action in the House, where he is a key player, but noted "solid Democratic encouragement" for him to make the race.

*Vermont: Ex-governor Richard A. Snelling, the GOP's main hope against Sen. Patrick J. Leahy (D-Vt.) has said nothing. But Snelling has grown a beard and has left for a trans-Atlantic sailing expedition, signs some GOP leaders in the state take as discouraging to the prospects of his candidacy.

*Wisconsin: With Gov. Anthony S. Earl (D) running for reelection and Rep. David R. Obey (D-Wis.) saying he is "very much inclined against" making a Senate race, the field of prospective opponents to Sen. Robert W. Kasten Jr. (R-Wis.) includes no one with experience in statewide office or Washington.