There are now more registered Democrats than Republicans on Southampton Town’s voter registration rolls—possibly for the first time in the town’s long history as a Republican stronghold, dating back to the 19th century.

After years of slowly closing the gap, the Democratic Party has nudged just slightly ahead, according to the most recent Suffolk County Board of Elections figures: there are 12,749 registered Democrats, compared to 12,702 Republicans. Another 3,164 are registered third-party voters—including 2,183 Independence Party members. And the number of unaffiliated voters, 10,609, is nearly as high as either major party.

The Democratic Party jumped into the lead with an increase of more than 1,000 new registered voters for the party this year, compared to last year’s 11,719 registered Democrats in the town and 12,078 registered Republicans.

A look back to 2011 shows steady climb for the Democrats: The gap went from a 1,631 difference in registered voters in 2011 to a 359 difference in 2016, according to Suffolk County enrollment figures. A surplus of unaffiliated voters also played a part in the slimming gap between the two major parties—as the number of unaffiliated registered voters has increased by 1,071 since 2011.

“As of the end of December, that was the first time in Southampton history,” said Southampton Town Democratic Committee Chairman Gordon Herr, regarding his party’s newly acquired dominance in the town. “It’s quite an achievement.”

Mr. Herr cited a number of different reasons for the recent Democratic spike, including the party’s grassroots efforts to register new voters outside grocery stores and other public centers. He also credited new, younger voters registering for the first time in the town, as well as second-home owners choosing to register in the town as Democrats.

Southampton Town Republican Committee Chairman Damon Hagan seized on the latter point, saying that adding second-home owners to the Democratic ranks was taking away the “voice” of year-round residents.

“There has been a concerted effort by the Southampton Town Democratic Party to recruit people who do not live in Southampton to come out to the East End and register their vacation homes for the purposes of stifling the local community’s voice,” Mr. Hagan said. “We’ve seen it time and time again … to have what is usually a Republican town—the community’s voice quieted—and have Manhattan’s voice be louder.”

Mr. Herr argued that the town’s Democratic Party is more closely aligned with the residents than the Republican Party because of Democrats’ policies that support the environment and balanced approaches to the economy and development. “We have a very strong Democratic Party in Southampton Town,” he said. “It’s probably the strongest Democratic Party in Suffolk County. Our values drive … a lot of people who live out here.”

Looking to the future, Mr. Hagan said the town’s Republican Party will continue community outreach and education about the issues, as well as putting up a “stellar slate” of candidates in the November election.

As for Mr. Herr, he said he anticipates a continued increase in registered Democrats in the town due to his party’s “grassroots campaigning” as well as increased unrest resulting from the recent presidential election. “I think the trend is going to continue,” he said. “I think especially now with the current state of national politics. I think there are a lot more people registering Democrat.”