PIERRE - Sen. Tim Johnson has filed his official nominating petition to seek a third term in the U.S. Senate.

The Democratic senator's nominating petition was filed as of Wednesday, according to Secretary of State Chris Nelson's Web site.

State Rep. Joel Dykstra of Canton also filed petition signatures to seek the Republican nomination to challenge Johnson. An unknown candidate, Charles Lyonel Gonyo of Trent, also filed to run in the GOP Senate primary.

Two other Republicans have said they will seek the Republican nomination to run for the U.S. Senate. They are self-employed businessman Sam Kephart of Spearfish and retired real estate agent Bert Tollefson.

The deadline for filing nominating petitions for congressional and state races is Tuesday.

Johnson's bid for a third term has attracted relatively little attention as South Dakotans have focused on his recovery from a December 2006 brain hemorrhage. Republicans have generally avoided publicly criticizing Johnson, who continues to undergo speech and physical therapy.

Dykstra said he is clearly the front-runner in the GOP Senate primary since Gov. Mike Rounds and Sioux Falls businessman Steve Kirby decided not to enter the race.

"Our goal when we started the petition process was to collect signatures statewide. This week's results exceeded our goal and illustrate that hard work and devoted volunteers can make a real difference in a campaign," Dykstra said in a written statement.