A New York state Democratic Party effort to get out the vote could backfire. The mailer scores residents on their voting history and tells them, ‘If you do not vote this year, we will be interested in hearing why not.’ Allison Pasek/The Wall Street Journal

Democratic Party postcards and letters sent to more than 800,000 New Yorkers this week gave them grades based on how often they had voted and told them their voting records in 2014 would be watched.

Political observers, both partisan and not, said the mailers were akin to a pre-Halloween scare.

But so-called voter shaming has become standard nationwide, with recent examples in Alaska, North Carolina and Virginia, Democratic Party officials say. The mailers have proven particularly effective in competitive races, increasing voter turnout by as much as eight percentage points, according to a Yale University study in 2008.

The New York mailers, sent by the state Democratic Committee, went to voters who were seen as strong party supporters but “not always reliable” when it came to casting ballots, a Democratic official said.

About 75% of the mailers were sent to voters in New York City. Depending on their voting histories, recipients received one of four grades: excellent, good, fair or incomplete.

“This flier is part of the nationwide Democratic response to traditional Republican voter suppression efforts because Democrats believe our democracy works better when more people vote, not less,” said Peter Kauffmann, a spokesman for the state Democratic Committee.

David Laska, a spokesman for the New York state Republican Committee, said the mailers “implicitly threatens voters to drive turnout,” calling it “beyond the pale” and “inexcusable.”

He said the state Republican Party hadn’t adopted similar tactics.

While in other states individual candidates have used such mailers, that isn’t true for the New York gubernatorial race, which pits incumbent Democratic Gov. Andrew Cuomo and Republican Westchester County Executive Rob Astorino.

Mr. Cuomo, in Syracuse on Friday, offered no criticism of the mailers. “I haven’t seen the mailer so I don’t know exactly what it is, but it’s my understanding that…there’s nothing new about this or novel about this,” he said. “But I don’t see how it can be bad when you’re telling people, exercise your franchise.”

The language used in the mailers bothered some Democratic Party officials. The end of one of the letters reads: “If you do not vote this year, we will be interested to hear why not.”

“I think that last line has a threatening tone to it,” said Jay Jacobs, chairman of the Nassau County Democratic Committee in Long Island, where some of the mailers were sent. “I don’t think that’s the way we go about it.”

Analysts said the mailers could backfire.

“It’s just dumb,” said Gerald Benjamin, a professor of political science at SUNY-New Paltz. “Citizens rightly believe their vote is a private matter and this could cause some voters to see this as an excessive scrutiny of their behavior.”

—Erica Orden contributed to this article.

Write to Derek Kravitz at Derek.Kravitz@wsj.com