Marion Michael Rounds (born October 24, 1954) is an American businessman and politician serving as the junior United States senator from South Dakota since 2015.

Rounds ran for governor of South Dakota in 2002, and after an upset victory in the Republican primary, defeated Democratic nominee Jim Abbott.

[9] As the 2002 race for governor took shape, media and political observers largely dismissed Rounds as an extreme long shot.

When Thune passed on the race to challenge Senator Tim Johnson, state Attorney General Mark Barnett and former Lieutenant Governor Steve T. Kirby became candidates.

[14] After winning the Republican nomination, Rounds chose State Senator Dennis Daugaard of Dell Rapids as his running mate.

Their Democratic opponents were University of South Dakota President Jim Abbott of Vermillion and his running mate, former State Representative Mike Wilson of Rapid City.

The results were as follows:[17] Two Democratic candidates emerged to challenge Rounds: Jack Billion, a retired surgeon and former state legislator from Sioux Falls, and Dennis Wiese, the former president of the South Dakota Farmers Union.

Billion easily won the nomination and selected Rapid City school board member Eric Abrahamson as his running mate.

[21] On February 22, 2006, the state legislature of South Dakota passed an act banning all medical abortions except those necessary to save the mother's life.

[33] Speculation persisted that in 2014 Rounds would seek the United States Senate seat held by Tim Johnson, a Democrat who had served since 1997.

October 2014 polls showed a closer three-way race between Rounds, Weiland, and independent former Senator Larry Pressler.

The results were:[40] In the 2020 election, Scyller Borglum, a first-term member of the South Dakota House of Representatives, challenged Rounds in the Republican primary.

On May 15, 2024, the group released a report recommending actions for the federal government to take to regulate and support AI development.

[48] In 2017, Rounds was one of 22 senators to sign a letter[49] to President Donald Trump urging him to withdraw the United States from the Paris Agreement.

[50] Rounds supported embattled Environmental Protection Agency Administrator Scott Pruitt, who had come under scrutiny because of extraordinary expenditures for personal security and luxury travel, and the appearances of ethical conflicts, defending him on Meet the Press.

"[51] Rounds opposed the FIRST STEP Act, a bipartisan criminal justice reform bill that Trump signed into law.

[56] On January 9, 2022, Rounds said that the 2020 presidential election was not stolen from Donald Trump: "[We] looked at over 60 different accusations made in multiple states.

Rounds said the election was fair, and added that Republicans should stop making arguments to the contrary: "If we simply look back and tell our people, 'Don't vote because there's cheating going on,' then we're going to put ourselves in a huge disadvantage.

"[57] Trump responded by calling Rounds a "jerk", "crazy" and "stupid" and accused him of being "woke" for acknowledging the election results.

In May 2011, Rounds's alma mater, South Dakota State University, gave him an honorary doctorate for public service.