The Secretary of State has issued press releases about the number of voters switching their registratoins to the Democratic or Republican Parties presumably to participate in their contested primaries. Here are the changes in party registration from August, 2015 to the primary election of May 2016.
Democratic 818,399 to 932,892 (change) 114,493 + 14%
Republican 643,928 to 679,889 (change) 35,961 + 5.6%
Non Affiliated 530,061 to 679,889 (change) (46,795) (8.8%)
Ind. Party 109,681 to 103,353 (change) (6,328) (5.8%)
While it seems like an unusual wave of voters to the Democratic and Republican parties, compare these changes to the changes from August 2007 to May 2008 the last time Oregon had a contested primary, though it was only the Democratic Primary that was contested, unlike this year when both Democratic and Republican Primaries appeared to the contested until just yesterday.
(Notes: The IPO was a nascent party in 2007 so it’s number is irrelevant. In 2008 some members of the Pacific Green Party split and formed the Progressive Party of Oregon. I noted the Pacific Green Party in this table as Progressive+ Green so that the historical comparisons are more informative)
The Democratic Party’s increase in membership is consistent with contested primary POTUS years. The GOP membership increased when there was a contested primary (2016) but decreased when there was no contested primary (2008)
The left leaning minor parties are losing more members in 2016 than they did in 2018. Factors may include the fact that Sanders is a more attractive candidate than Obama was in 2008 to the far left, as well as the fact that motor voter is taking a heavy toll on party membership across the board. And these two factors are taking a heavy toll on the left leaning minor parties.
The Libertarian Party seems to regularly have about 10% of it’s members re-register (as Republican I assume) to participate in the POTUS primary, then those voters re-register as Libertarians later. They appear to have a solid loyal party base.
If you assume approximately 60% of NAV’s lean left and 40% lean right, then you can infer that Non affiliated voters are re-registering with a major party in about the same numbers for 2016 (with a contested GOP primary) as they did in 2008 (when only the Democratic primary was contested)
Though not reflected in this table, the IPO announced this week that over 20,000 NAV’s had requested an IPO ballot. So, even though the IPO lost over 6,000 registered member, it will have 20,000 more voters who will be participating in it’s primary election. If you added those 20,000 voters into the IPO’s May registration totals, the IPO will have an actual increase of 13,000 voters, or about 12%.