The South Dakota Secretary of State’s web page keeps a running tally of the number of registered voters in each party. Here is a link to the totals as of July 8, 2017. The percentages are: Republican 46.21%; Democratic 30.68%; Libertarian .31%; Constitution .09%; other and independent 22.72%.
Just before the November 2016 election, the percentages were: Republican 46.31%; Democratic 31.35%; Libertarian .30%; Constitution .09%; other and independent 21.95%.
What you are seeing is ordinary changes in registration following an election year.
New registrants are those picked up when they get a drivers license, while people who die are dropped from the registration lists. Older people, who are more susceptible to death, are more likely to have a partisan affiliation. So the Democrats lose ground as old Democrats die with fewer replacements, Republicans tread water, and nonaffiliated gain, as new voters don’t affiliate, particularly if they only registered because it was easy to do while getting a license.
The number of inactive voters had a dramatic decline (25% in the last month). This is due to the purge of inactive voters who did not vote in 2014 and 2016. To be inactive, there has to be a loss of contact AND then to skipped federal elections. So these were persons recognized as missing back in 2012 and 2013. They were placed on the suspense list then, and completely removed after missing the next two elections. But they were removed from the partisan totals back in 2013.
In the next few months, there will be an increase in inactive voters, and decline in active voters, as the voting rolls are prepared for 2018. This will tend to impact Democratic voters the most, since they tend to have less economic stability, and may have moved away. In many states this would tend to have an impact on the unaffiliated and minor party voters. But in South Dakota, these phenomena are so new, that new registrants may make up the loss.
The real significance of this new South Dakota tally is that the Secretary of State recognizes the Constitution Party as ballot-qualified. In theory the Secretary of State might have done something hostile, because HB 1034 changed the definition of “party” from a group that got 2.5% for Governor, to a group that got 2.5% for any statewide race. That was a liberalizing bill, but it did have the effect of forcing groups to pass the vote test every 2 years instead of every 4 years. The Constitution Party didn’t get 2.5% for any statewide race in 2016, although it easily could have if it had run for Public Service Commissioner. HB 1034 went into effect July 1, 2017, so at that moment there was a theoretical danger. To enforce it now against the Constitution Party would have violated due process, and would have been terribly unfair, because when the Constitution Party decided not to run anyone for Public Service Commissioner in 2016, there was no legal reason for it to do so. It could not have known that the law would change in 2017.
I was commenting on the reason for change which your post appeared to be highlighting.
Darrell Castle only got about 1% of the presidential vote. I don’t think it is a safe conclusion that a Constitution candidate would easily gain 2.5% of the vote for US Senator, US Representative, or Public Service Commissioner.