Democrats settled their gubernatorial nominations yesterday in Maine and South Carolina, two states that are high on the Republican target list for takeover in November.

Lt. Gov. Mike Daniel (D), the favorite, led in South Carolina but failed to pull enough votes to avoid a June 24 runoff against former Hilton Head developer and Winthrop College president Phil Lader.

Early today, however, Lader announced he would ask people to support Daniel in the interests of party unity. "I've made a decision not to run against Mike Daniel," Lader said in Columbia. "The results today have been clear. There will be a runoff only because the law requires it."

In Maine, Attorney General James Tierney (D) defeated four rivals for the nomination. He will face Rep. John R. (Jock) McKernan Jr. (R), a moderate, who rolled up a 2-to-1 margin over the former director of President Reagan's Maine campaign, Porter D. Leighton.

McKernan's House colleague, Rep. Carroll A. Campbell Jr. (R), who has close ties to the president, drew no intraparty opposition for governor in South Carolina.

Maine and South Carolina are two of 13 states where incumbent Democratic governors have chosen or are required to step down, opening seats to possible GOP takeover.

Maine Gov. Joseph E. Brennan was unopposed for nomination to McKernan's House seat and is favored to win it in November. South Carolina Gov. Richard W. Riley (D) is headed for retirement after declining to challenge Sen. Ernest F. Hollings' (D-S.C.) bid for a fourth full term.

Maine voters soundly defeated a fundamentalist-backed referendum that would have imposed tough state penalties on sellers of pornography -- giving a 2-to-1 victory to civil libertarians who said it smacked of censorship.

In a special election to fill the seat of Rep. Joseph P. Addabbo, who died in March, only 41 votes separated the leader, the Rev. Floyd Flake, a prominent minister and independent Democrat, and state Rep. Alton R. Waldron Jr., the endorsed Democrat. The winner will be the first black congressman from Queens.

In South Carolina, with 96 percent of the precincts counted, Daniel, Riley's partner in education and economic development projects the past four years, had 47 percent of the vote, slightly under the 50 percent he needed to avoid a June 24 runoff. Lader, running as an anti-politician reformer, had 27 percent. Retired Circuit Court Judge Frank Eppes, who conducted a folksy, friends-and-neighbors campaign from an 18-wheeler, got 18 percent, and state Sen. Hugh Leatherman, 9 percent, with a largely self-financed effort.

In a second South Carolina contest with important implications for November, Secretary of State John T. Campbell (D) was on his way to a 53-to-47 percent victory over State Human Rights Commisioner Jim Clyburn, who was bidding to be the first black to win a Democratic nomination for statewide office. Many Democratic strategists had said they needed Clyburn on the ticket to assure a large black turnout in November.

In the Maine Democratic race, with 65 percent of the precincts counted, Tierney led lawyer-lobbyist Severin M. Beliveau, a longtime Democratic power-broker, 40 to 24 percent, with state Sen. G. William Diamond at 21 percent, former Brennan aide David E. Redmond at 12 percent and race track owner Joseph J. Ricci at 3 percent.

Republicans have not won the Maine governorship since 1962, but they see in McKernan, a 38-year-old, second-term congressman, their strongest contender in a generation.

South Carolina Republicans feel the same way about Campbell, 45, who came to Congress in 1978 and in 1980 spearheaded Reagan's successful South Carolina campaign. Campbell faced a challenge for the nomination from Rep. Thomas F. Hartnett (R-S.C.), but rather than risk splitting the party between its Greenville and Charleston factions, Hartnett finally agreed to run for lieutenant governor with Campbell. Republicans did face a contest for the nomination against the strongly favored Hollings, with former U.S. attorney Henry McMaster drawing establishment support and Henry S. Jordan, a surgeon, carrying the endorsement of the Rev. Marion G. (Pat) Robertson and the hopes of many of the state's Christian activists. McMaster won by a 53 to 47 percent margin.

In Charleston, County Councilman James Stuckey and ex-representative Mendel J. Davis matched votes in a Democratic congressional primary and headed for a runoff to see who will challenge state Sen. Arthur Ravenel for the seat Hartnett is leaving. Republicans face a runoff for the Campbell vacancy.

South Dakota, the other state voting yesterday, had no major contests on the primary ballot. State tax commissioner Kent Conrad was unopposed for the Democratic nomination against Sen. Mark Andrews (R-N.D.) and Bismarck financial consultant Syver Vinje was the only Republican who filed against Rep. Byron L. Dorgan (D-N.D.) for the lone House seat.