By Steven Adams

CHARLESTON, W.V., Jan 30 (Reuters) - A West Virginia sheriff pleaded guilty to voter fraud in the state's 2010 primary election including illegally filling in some absentee ballots out of fear he might lose a close election, authorities said on Monday.

Lincoln County, West Virginia Sheriff Jerry Bowman admitted falsifying absentee ballots in a case stemming from an investigation by federal authorities, the U.S. Attorney's office said.

Also pleading guilty to lying to investigators was Lincoln County Clerk Donald Whitten, the U.S. Attorney said.

"I want the case to send a simple message: you cannot steal election in southern West Virginia," said U.S. Attorney Booth Goodwin. "Honest elections are worth fighting for."

Bowman, 58, admitted falsifying more than 100 absentee ballots, was illegally in the same room as voters marking absentee ballots and sometimes marked the ballots himself, authorities said.

He was running for circuit clerk at the time and concerned about a close election, court documents said.

Bowman appeared to have won the primary but about 200 contested absentee ballots were later thrown out, making incumbent Circuit Clerk Charles Brumfield the victor.

Whitten, 62, admitted to making a false statement about his roll in the conspiracy, in which he provided absentee ballots. He faces up to five years in prison and a $250,000 fine.

Both men, who are Democrats, also agreed to resign from office and never seek office again.

The pair also agreed to cooperate as the U.S. Attorney's office and West Virginia Secretary of State continue their investigation into the southern West Virginia elections.

Neither man has been arraigned yet.

Several years ago, Lincoln County's Circuit Clerk and Assessor were convicted of felonies for vote buying after tampering with the 2004 Democratic primary. That same assessor, Jerry Weaver, is running for sheriff, despite being a felon.

(Editing By Ellen Wulfhorst and Greg McCune)