WILLIAMSON, W.Va. (AP) _ A special grand jury that charged 15 Mingo County politicians with buying slots on election ballots struck a blow against the corrupt practices that dominated elections in southern West Virginia for years, a prosecutor says.

The practice of ''slating'' candidates and encouraging citizens to vote straight tickets has contributed to the conditions alleged in an indictment that was handed up Thursday, said special prosecutor James Colburn.

''I don't see how after this anyone could run as part of a slate,'' Colburn said.

Those indicted included a state senator, the county prosecutor, the mayor of Williamson, two county commissioners, two county school board members, three former delegates to the state assembly, one magistrate and one former magistrate, a former court clerk, a former county clerk and a former school board member who was also director of the county anti-poverty agency.

The state grand jury charged the 15 with paying to be listed on Democratic slates controlled by former Sheriff Johnie Owens and with using bribes and illegal campaign contributions to ''subvert the free election system.'' The indictment focused primarily on the 1984 election.

A 16th defendant, state Sen. Truman Chafin, was indicted on a felony bribery charge. He is accused of accepting a bribe when he was a county commissioner to help current Mingo County Sheriff Charles ''Eddie'' Hilbert ''buy'' the office from Owens for $100,000.

''I've tried all my life to live a decent life, and something like this is quite a shock,'' said one indicted man, Robert Simpkins, a member of the county school board.

''I can't remember if I gave them any money, but it couldn't have been any more than $500,'' said another of the accused, former County Clerk Hugh Wellman. ''I didn't think there was anything illegal about it. You go through life, and you forget these things. That's been four years ago,''

Colburn said the indictment should be a warning to other politicians who might use slates to further their positions. The grand jurors, however, blamed residents of Mingo County for the dominance of slate politics.

''This Grand Jury would urge that every citizen of Mingo County not sell or give up their franchise or right to vote, but to exercise it without fear or purchase,'' the jury's report said.

Former magistrate Bill Webb said the indictments ''were blatantly political and were returned without any evidence whatsoever.'' He said he will continue his re-election campaign.

Also indicted Thursday was Goodwill Motors Inc., a used car dealership at Delbarton owned by former county commissioner and former county Democratic Chairman Stewart Justice. Justice was not named as a defendant.

Arraignment for all defendants is scheduled Wednesday in Mingo County CircuitCourt.

Owens and Hilbert were not named in Thursday's indictments.

Owens pleaded guilty earlier this year in federal court to one count of bribery and three counts of tax evasion and is scheduled to be sentenced on those charges April 18. He faces sentencing on a state income tax evasion count April 19.

Hilbert has been indicted by a federal grand jury on 13 felony counts of buying his office, helping protect a drug ring in return for illegal campaign contributions and buying and selling marijuana out of his office. He is scheduled to enter a plea to those charges on April 18.

Colburn said the state grand jury will meet at least once more, on May 10, the date of the West Virginia primary, to hear evidence of any violations in that election.