Rep. Thomas F. Hartnett (R-S.C.) was losing the first set in a match at the Charleston Tennis Club two weeks ago when the phone call came in from Camp David.

"Obviously, it was an orchestrated thing," said Hartnett, recalling that earlier he received similar calls from Senate Majority Leader Robert J. Dole (R-Kan.) and Sen. John Heinz (R-Pa.), the chairman of the National Republican Senatorial Committee. "I mean, President Reagan wasn't sitting up there at Camp David and all of a sudden saying, 'Hey, I think I'll call Hartnett.' "

Reagan called to urge Hartnett to challenge Sen. Ernest F. Hollings (D-S.C.) in next year's congressional elections. He told Hartnett how important Republican control of the Senate had been to his presidency, urging the legislator to rethink his tentative decision not to take on the entrenched Hollings. The two men ended up talking about baseball.

In this case, Reagan's noted powers of persuasion failed him. Hartnett announced that he will leave Congress to run for lieutenant governor next year. After three terms in the House, he said, he is disgusted with the ways of Washington and had concluded he had little chance of beating Hollings.

But the fact that the president, late on an autumn Saturday afternoon, would place a call to a little-known House member was a sure sign that a certain political season is reaching its peak. It is the recruiting season, when the campaign committees of the two parties in Congress reach out across the country in search of political talent. Like college football coaching staffs in hot pursuit of high-school prospects, they are now putting together the lineups of the future, the ones that will clash head-on in the House and Senate campaigns of 1986.

During the August congressional recess, Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee, spent 10 days on the road, visiting eight midwestern states to talk to local Democrats about prospects in House races. The Midwest, where Reagan's farm policies are increasingly unpopular, is a region where the Democrats say they think they can make gains.

"I try to get them excited," Coelho said.

Last week, Steve Nix, Midwest field director for the National Republican Congressional Committee, flew to Lansing, Mich., where he met separately with two GOP House prospects, Jim Dunn and Tom Ritter. In 1982, Dunn, then the Republican incumbent, lost his House seat to Rep. Bob Carr (D-Mich.), from whom he had won it two years earlier. He then lost the Republican Senate primary in 1984.

Last year, Ritter mounted a strong but unsuccessful challenge to Carr. Now, both men are thinking about 1986, so Nix flew to Lansing to hear firsthand about their plans, hoping that a potentially costly GOP primary fight in the district can be avoided.

In such forays, and in a steady stream of visits to Washington by potential candidates, the lineups for 1986 are quietly taking shape. The Senate, where continued Republican control will be at stake, is more visible, often attracting well-known figures such as former Vermont Gov. Richard Snelling (R), who was called to the White House for a meeting with Reagan before he announced he will challenge Sen. Patrick J. Leahy (D) next year.

But it is in the House, where all 435 seats are up for election, that the recruiting activity is most feverish, centered in the two congressional campaign committees.

For House Republicans, 1986 presents a less than attractive prospect. The magic Reagan name will not be at the top of any ballots, and historically the party of a second-term president has suffered serious -- sometimes massive -- losses in the off-year congressional elections.

But, according to GOP campaign officials, the lessons of history have not seriously diminished Republican enthusiasm after Reagan's landslide reelection.

"Everybody knows that this election could be a disaster, and that makes them work very hard," said Joseph R. Gaylord, executive director of the Republican campaign committee. "There is a strong belief that if we get by this election without losing 25 seats, it will break the back of the Democrats' hold on the House."

For their part, the Democrats are busy lowering expectations, predicting a pickup of 10 to 15 seats next year. The reason for this historically modest goal, they say, is that, in holding their 1984 losses to 14 seats in the face of the Reagan landslide, they limited their opportunities in the next election.

"We are the victims in 1986 of our success in 1984," said Mark Johnson, the communications director of the Democratic campaign committee. "We can't win back seats we didn't lose. The Republicans also have a lot of money, and they are very good at damage control."

Damage control, meaning the protection of vulnerable incumbents, is the top priority of both campaign committees. But with most House seats considered safe, the overall contest is likely to be decided in fewer than 100 congressional districts, including about 40 that are expected to be open because of retirements and decisions by incumbents to run for other offices.

Putting together the strongest possible lineup for the key contests is a process with no real beginning or end, but which comes to a head about this time every two years as candidate filing deadlines approach. Before the polls had closed on election day in 1984, Coelho was working on the next round, placing calls to Democrats who had run strong campaigns he knew would fall just short. His message: You ran a good race, start thinking now about 1986, you can count again on strong support from the Democratic Congressional Campaign Committee.

In several cases, Coelho was successful, setting up 1986 rematches of close House races. These include three House races in North Carolina, a state in which the Democrats lost four close contests in 1984 and which has been targeted by the Democrats next year.

Robin Britt, who won 49 percent of the vote against Rep. Howard Coble (R-N.C.), and James M. Clarke, who got 49 percent against Rep. William M. Hendon (R-N.C.), are ready for another round. The third North Carolina rematch involves a challenge by D.G. Martin, who lost in 1984 to Rep. J. Alex McMillan (R-N.C.) by 321 votes.

Meanwhile, Coelho, and his GOP counterparts, have been around the country on recruiting trips. "I'm looking for somebody who is positive, who feels he can win, can raise money and has a rapport with people," Coelho said. "I'm not looking for liberals or conservatives. That's not my bag. I'm looking for winners."

At the National Republican Campaign Committee, Edward A. Goeas III, the field director, follows the same credo. He said he looks for candidates with their own base in the district who have some name identification, who are articulate and who can raise money.

Armed with mountains of research material on political and demographic trends and incumbent Democratic strengths and weaknesses, GOP field workers under Goeas' direction are combing the congressional vineyards, particularly in open districts and about 40 Democratic districts that the GOP expects to target for major campaign efforts in 1986.

Despite the historical trend, Goeas said candidate recruitment has been easier than it was before the 1984 congressional elections, when the memory of the 26 seats the GOP lost in 1982 dampened enthusiasm.

Goeas, who has dubbed himself "Mr. Negative" at the campaign committee, said he stresses the obstacles Republicans face in 1986, but with little apparent impact on recruitment. "You can see it in their eyes," he said of potential candidates. "They think they will be the exception."

As the pre-election year jockeying goes on, with the two campaign committees eyeing each other suspiciously, the two parties approach 1986 with clear strengths and weaknesses. The Republicans have more money, better organization at the national level and a still popular second-term president in the White House. The Democrats have history on their side and a larger field of incumbents, giving them an important beginning edge. Democratic control of the House will not end next year, but if the Republicans are to reach their goal of a House majority in 1992 they cannot afford massive losses in 1986.

Goeas promises "some surprises" in Democratic-held districts, but Coelho said: "There is no reason an incumbent should lose. If we do our job, we maintain control of the House for the foreseeable future."