Democratic incumbent: Tim Johnson
Republican candidate: Joel Dykstra
Johnson, 61, has served in the Senate since 1997. He earlier was in the U.S. House for 10 years and, prior to that, the state legislature.
Dykstra, 50, has served in the state House of Representatives since 2003.
Real Clear Politics reports a 22-point poll lead for Johnson.
Johnson has overcome multiple health issues. He has been treated for prostate cancer and, in 2006, he suffered a stroke that left him incapacitated for several months. He has recovered and has resumed his Senate duties full time.
Johnson's two earlier elections to the Senate were by narrow margins. His junior colleague, John Thune, is a Republican. Thune came within 524 votes of ousting Johnson in 2002, before going on to unseat Senate Democratic Leader Tom Daschle in 2004.
Johnson's comfortable position in this year's campaign can seemingly be attributed to the overall Democratic trend, and the lack of a high-profile Republican opponent.