Until the middle of last fall, David Johnson, executive director of the Democratic Senatorial Campaign Committee (DSCC), would tell anyone who'd listen that Indiana was going to be the "sleeper" Senate race of 1986.

Then the DSCC flew the candidate it had recruited to run uphill against Sen. Dan Quayle (R-Ind.) -- a feisty Indianapolis state senator named Louis Mahern -- to a challengers' retreat weekend on Maryland's Eastern Shore. A day into the session, Mahern had a heart attack. He later decided, for health reasons, not to run.

Scratch Indiana from the Democrats' hot-race list.

The misfortune illustrates the unpredictability of that least glamorous of political chores -- candidate recruitment -- and it served as a bit of a bad omen for the Democrats as they headed into the tail end of their recruiting season.

Since then, Democratic hopes of winning back control of the Senate this year (they need a net pickup of four seats) have been tempered by recruiting failures in three key states -- New York, North Carolina and North Dakota -- where, but for the want of the "right" challenger, a Republican seat had seemed ripe to be plucked.

No pattern connects the failures: In heavily Democratic New York, first-term Sen. Alfonse M. D'Amato (R) will have to defend the seat he captured in 1980 with just 45 percent of the vote. But the two strongest potential Democratic challengers -- former representative Geraldine A. Ferraro and Brooklyn District Attorney Elizabeth Holtzman -- both begged off. The only Democrat to come foward so far is Mark Green, a former lieutenant of Ralph Nader's and former speechwriter for Sen. Gary Hart (D-Colo.). Green lost his only previous political race -- for Congress -- in 1980.

In North Carolina, where Sen. John P. East (R-N.C.) is stepping down for health reasons, a bitter primary on the Republican side is already under way, pitting moderate Rep. James T. Broyhill (R) against conservative David Funderburk, the choice of of Sen. Jesse Helms' shadow state Republican Party, the Congressional Club. But the Democrats appear unready to profit from the infighting. Former governor James B. Hunt (D), still smarting from his defeat at the hands of Helms in 1984, was the first to say no. Then, Rep. Charles Rose (D), Duke University President and former governor Terry Sanford, and University of North Carolina Chancellor William Friday, all followed suit. Four Democrats are talking about making the race; none of them have statewide stature.

In North Dakota, the DSCC commissioned a poll last summer that showed the state's only congressman, Rep. Byron L. Dorgan (D), would defeat first-term incumbent Sen. Mark Andrews (R) by 57 to 32 percent. It thereupon collected a dozen Democratic senators in a room in the Capitol, sat them down with Dorgan and the poll results, and had everyone pitch the virtues of being a senator. Dorgan listened politely and told the group thanks, but no thanks. He indeed wants to be a senator, but prefers to wait until 1988 when the incumbent of his own party, Sen. Quentin N. Burdick, may retire. Meantime, the Democratic candidate for 1986, state tax commissioner Kent Conrad, is considered a long shot against Andrews.

"Because of those three situations that didn't develop, the Democrats' chance of winning the Senate is a little under 50 percent right now," said Mark Gersh, a political anaylst for the National Committee for an Effective Congress (NCEC), which funds progressive candidates. "It they'd had the right candidates, it would be 75 percent."

Republicans enter the new year feeling as if they dodged a bullet. "We come into 1986 stronger than we went into 1985," said Thomas Griscom, executive director of the National Republican Senatorial Committee. "I was especially nervous about North Carolina, but the Democrats struck out," added Sen. John Heinz (R-Pa.), the NRSC chairman.

Even though Republicans must defend 22 of the 34 seats at risk this year, Heinz said he believes his party will break even or suffer a net loss of just one. His Democratic counterpart, Sen. George J. Mitchell (D-Maine), chair of the DSCC, says there's a better than even chance the Democrats will get their four.

Mitchell has had his share of recruiting successes, too. Rep. James R. Jones (D-Okla.), is set to take on freshman Sen. Don Nickles in Oklahoma, where a troubled oil and farm economy may make the seat hard to hold. Outside of Gov. George Nigh (D), who decided not to make the challenge, the hard-hitting Jones is considered the strongest Democratic candidate.

In Florida and Idaho, popular Democratic governors are off and running for the Senate. Florida Gov. Robert Graham and Gov. John V. Evans of Idaho are taking on first-term incumbents, each of whom won narrow victories in the Reagan sweep of 1980: Sen. Paula Hawkins (R-Fla.) and Sen. Steve Symms (R-Idaho).

Hawkins started her race well behind in the polls, then narrowed the gap with a course of early televisions ads in November. But a poll by the Miami Herald published Dec. 11 had Graham with a 51 to 35 percent lead (other polls show a closer race). In Idaho, Symms has a narrow lead in early polls, but most anaylsts rate the race a tossup.

Heinz's recruiting problem has been convincing popular governors not to run for the Senate. He succeeded in Pennsylvania, where Gov. Richard L. Thornburgh (R) chose to forego a challenge to Sen. Arlen Specter (R-Pa). Specter will face the winner of a Democratic primary whose leading candidates are Rep. Bob Edgar (D), a liberal from the Philadelphia suburbs, and state Auditor General Don Bailey, a conservative from the west. Both are considered strong challengers.

In South Dakota, Gov. William J. Janklow (R) says he will announce Feb. 22 whether he will take on Sen. James Abdnor (R). Abdnor tried to keep the mercurial governor out of a primary by running television ads last fall that lifted his poll numbers to the point where he was for the first time leading the expected Democratic challenger, Rep. Thomas A. Daschle (D-S.D.). But Janklow seems unlikely to take the hint and headed for a primary. Either way, this seat represents a good chance for a Democratic pickup.

Of the 16-member GOP class first elected to the Senate in 1980, five have reason to expect a relatively quiet reelection year: New York's D'Amato, Iowa's Charles E. Grassley, Indiana's Quayle, Alaska's Frank H. Murkowski and New Hampshire's Warren B. Rudman, who is expected to announce this month he'll seek reelection.

The others face varying degrees of exposure, although as a group they appear in better shape than their detractors would have guessed. Most are following the Grassley model of survival: take credit for the Reagan administration's economic successes but keep a wide berth of the president -- on whose coattails they all swept into office -- when it comes to farm or trade policy.

Washington state's freshman, Sen. Slade Gorton (R), knows who his challenger will be -- former Carter administration secretary of transportation Brock Adams. Gorton starts as the favorite but many think the race will be tight.

Others of his classmates await Democratic primaries to sort out their opponents. In Georgia, after several top state Democrats declined to get into a race against Sen. Mack Mattingly (R), three contestants are emerging: Rep. Wyche Fowler Jr. (D) of Atlanta; former Carter White House chief of staff Hamilton Jordan, who has just finished a round of cancer treatments, and Atlanta attorney David Garrett, a prominent supporter of Gary Hart's 1984 presidential campaign.

In Alabama, a sharp ideological primary showdown had loomed all winter between conservative Rep. Richard C. Shelby (D) and liberal Secretary of State Don Siegelman (D) for the right to take on freshman Sen. Jeremiah Denton (R). But party sources now say Siegelman is expected to run instead for state attorney general, clearing the way for Shelby. He will face an uphill fight in the fall.

In Wisconsin, freshman Sen. Robert W. Kasten Jr. (R) caught two breaks last year when Gov. Anthony S. Earl (D) and Rep. David Obey (D) passed up a race. A multicandidate Democratic primary is in the offing. Ed Garvey (D), assistant state attorney general and former National Football League players' representative, is thought to have an edge over his principal opponent, former state party chairman Matthew J. Flynn.

Kasten was the subject of investigative articles in the Milwaukee Journal last year, charging he didn't report on his Senate disclosure forms some income from a failed business venture that landed one of his partners in jail. He also received bad home-state press over last month's drunk-driving arrest in Washington, D.C. In a state that prides itself on clean government, those developments could hurt Kasten.

There will be six open-seat races this fall -- three currently Democratic and three Republican -- and the early handicapping is a mixed bag. The Democrats seem guaranteed of taking away at least one of the GOP seats (Maryland), but Republicans have a chance at taking all three Democratic seats (Louisiana, Missouri, Colorado).

In Maryland, polls show Rep. Barbara Mikulski (D) of Baltimore has jumped out to a handsome early lead over a primary field that includes Gov. Harry Hughes (D) and Rep. Michael Barnes (D). They are fighting for the right to go after the seat being vacated by Sen. Charles McC. Mathias Jr. (R). The Republicans haven't yet found a candidate.

In Louisiana, where Sen. Russell B. Long (D) is retiring, Rep. W. Henson Moore (R) starts out as the favorite over Rep. John B. Breaux (D) in an open-seat race that will offer a test of the strength of party realignment in the South.

In Missouri, where Sen. Thomas F. Eagleton (D) is stepping down, the matchup between former Gov. Christopher (Kit) Bond (R) and Lt. Gov. Harriett Woods (D), is rated a tossup.

In Colorado, where Hart is foregoing a reelection campaign, a Denver Post poll shows Rep. Ken Kramer (R) in a dead heat, 41 to 41 percent, against Rep. Timothy E. Wirth (D). Wirth is being attacked as the congressman who dismantled Ma Bell. Before the conservative Kramer makes the finals, he first must survive a tough primary battle against State Sen. Martha Ezzard (R) and businessman Terry Considine (R).

In Nevada, where Sen. Paul Laxalt (R) is retiring, Democrat turned Republican ex-representative Jim Santini is the likely GOP nominee, and party sources say he gives the GOP its best chance of holding onto the seat. He will run against Rep. Harry M. Reid (D).

In Arizona, conservative Rep. John S. McCain III (R) is the heavy favorite over former state corporation commissioner Richard Kimball, a consumer activist, for the seat being vacated by Sen. Barry Goldwater (R).

Republicans have had a couple of recruiting disappointments of their own. In South Carolina, they tried to talk Rep. Thomas F. Hartnett (R) into challenging Sen. Ernest F. Hollings (D), but Hartnett is running instead for lieutenant governor. Hollings' likely opponent is former U.S. attorney Henry McMaster of Columbia, S.C.

In California, where Sen. Alan Cranston (D) has been targeted by Heinz as "the most vulnerable Democratic incumbent of 1986," the GOP problem is not too few candidates, but too many. Reps. Bobbi Fiedler, William E. Dannemeyer and Edwin V.W. Zschau; state Sen. and former Los Angeles police chief Ed Davis; state party chairman Mike Antonovich, and supply-side economist Arthur Laffer have all announced, and no clear favorite has emerged. If it stays crowded and messy, and the winner takes only 30 percent of the vote, the primary won't be much of a springboard for the fall challenge to Cranston. On the other hand, Cranston's poll numbers have been under 50 percent all year.

The GOP got a boost in Vermont, where popular former Gov. Richard Snelling (R) will take on incumbent Sen. Patrick J. Leahy who is now the Democrats' most exposed incumbent.

In Ohio, Rep. Thomas N. Kindness (R) has begun running against Sen. John Glenn (D) and is hoping Glenn will be hobbled by a 1984 presidential campaign debt still in excess of $2 million. Glenn starts out as the favorite.

The rest of the Senate incumbents seem to be in for a waltz. Senate Finance Committee Chairman Bob Packwood (R-Ore.) escaped an expected challenge from Rep. Ron Wyden (D). Likewise, Majority Leader Robert J. Dole (R -- Kan) has gotten a bye from his strongest potential challenger, Rep. Dan Glickman (D).