His parents were firmly Republican, and he was in a blood red state, so it makes sense for him to become a Republican. How would his political career go? Does his views remain the same? Butterflies?
It would be a very different path for him. McGovern was a New Deal convert, supported Henry Wallace in 1948 and went back to the Democrats, serving as Executive Director of the South Dakota Democratic Party in 1953; he built the Democratic Party in the state before running for Congress and winning in 1956. The fact that he backed Wallace in 1948 makes it unlikely, without a major POD, that he'd be a Republican. Still, though, he did have a reputation for sticking to home state agricultural issues in the House and Senate and he was a formidable politician, managing to beat Joe Foss (a Medal of Honor winner) in his 1958 reelection bid. If he were to run as a Republican, he'd most likely be allied with the GOP's internationalist wing on foreign policy with a decided tilt toward the party's more liberal Eastern wing on social issues. In the 50s and 60s, that wouldn't be a particularly difficult place to be as long as he stuck to home state issues as he did in OTL. By the end of his Senate career, of course, he'd be out of touch with the Reagan wing of the GOP, but one could see him hanging on into the 1990s in the Senate. If he retired in 1998, he would have been 76 and perhaps one of the last great GOP liberals unless a primary took him out earlier.