The Libertarian Party now has approximately 500,000 registrants. It is the first nationally-organized party, other than the Democratic and Republican Parties, to have that many. The Libertarian Party had 411,250 registrants in February 2016, and in all previous years, had never had as many as 400,000.
The exact number will be known next week, when Pennsylvania furnishes the data.
The Constitution Party’s highest registration total had been 384,722 in February 2008. It declined sharply afterwards, because the California unit of the party, the American Independent Party, which had 85% of the party’s national registration, withdrew its affiliation with the national party in August 2008.
The highest Green Party registration was in October 2004, when the party had 312,963.
The American Independent Party of California also has more than 500,000 registrants. It has 507,377 in California. It is not a nationally-organized party, and its presidential nominee this year was Donald Trump. The current tally is the first tally at which the AIP has had as many as 500,000 registrants.
The Progressive Party of 1948 had 228,668 registrants in New York (under the American Labor Party label) and 22,461 in California, but very few in any other state.
Most states couldn’t furnish any statewide registration data before World War II. In many states, there was no such thing as voter registration except in large cities, so there were no statewide totals. Even as late as 1968, Ohio, for example, only had voter registration in urban counties.