Bob Mercer calculates that at least 41% of the 7,335 Republican voters newly registered since the June 2 primary are folks who were not previously registered to vote:
As of Thursday, July 23, there were 328,909 people registered to vote in South Dakota as Republicans. That compares to 321,574 who had registered as Republicans for the June 2 primary election.
Some of that 7,335 Republican increase appears to reflect people changing their voter registration to Republican for the July 28 runoff. Here’s an explanation:
— As of Thursday, July 23, there were 134,045 people registered as Democrats. That compares to 136,755 people registered as Democrats for the June 2 primaries. The result was a shift, or loss, of 2,710 Democratic-registered voters.
— As of Thursday, July 23, there were 155,432 people registered as independents and no-party affiliation. There were 156,991 people registered as independents and no-party affiliation for the June 2 primaries. That was a shift, or loss, of 1,559 independent and no-party registered voters.
— As of Thursday, July 23, there were 2,779 people registered as Libertarians. There were 2,806 people registered as Libertarians on June 1. That was a shift, or loss, of 27 Libertarian-registered voters.
In total, the people who previously were officially listed as Democrats, independents, no-party affiliations, and Libertarians fell by 4,296 between June 1 and Thursday, July 23. If all of those non-Republicans changed their registration to Republican and did it before the July 13 deadline, it would mean an additional 4,296 people would be eligible to vote in the Republican runoff.
It’s uncertain whether Doeden or Rhoden would get the majority of those new Republican converts.
Subtract the registration-switchers from the 7,335 increase in Republicans, and you get 3,039 new Republicans who hadn’t been registered at all in South Dakota before the June primaries [Bob Mercer, “A Surge of New Republicans Eligible to Vote in Runoff,” KELO-TV, 2026.07.24].
Look at those stats from the other direction, and you could say, “Holy cow! 59% of the new Republican registrants are party-switchers!” But the party-switching number is probably lower than that. The decrease in Democrats, independents, and Libertarians surely includes some natural attrition—folks dying, folks moving away, folks letting their registration lapse. Some significant percentage of those 4,296 D/I/L leavers aren’t around to register Republican. Republicans could not claim all of those leavers. Republicans also had to make up for their own natural attrition. So the percentage of newly registered Republicans who are first-time voters or whom the runoff has reëngaged after a long sit-out must be higher than 41%.