Only two-thirds of Americans say they are likely to fill out the 2020 Census, a number that has plunged in the past decade, according to a national study whose preliminary results were announced Thursday by the Census Bureau.

The study comes at a time when the 2020 count is embroiled in legal wrangling over the Trump administration’s controversial decision to add a citizenship question to the decennial survey.

The Census Barriers, Attitudes and Motivators Study finds 67 percent of adults saying they would be “extremely likely” or “very likely” to fill out the census form if they received it today.

That compares with 86 percent who said they were likely to fill out the form in 2008. Kyley McGeeney of PSB Research, who presented results at a Census Bureau event, cautioned that differences in how the surveys were conducted in 2008 and 2018 make it difficult to make an apples-to-apples comparison.

Still, the discrepancy is striking and may increase concern that the share of households that respond to the 2020 count — without costly in-person follow-ups — will fall short of expectations. The mail response rate in 2010 was significantly lower than the share of survey respondents who said they would participate.

The Census Bureau urged caution in comparing results from the two surveys, noting that the 2008 survey was conducted by phone and face-to-face while the latest survey was conducted by mail and online.

The three-month survey was conducted from February to April, a period that overlapped with the Census Bureau’s March 29 announcement that it would add a citizenship question to the 2020 questionnaire. As a result, the survey might have only partially captured the impact of the decision. Census experts have warned that the addition may scare off immigrants and their families, lowering the response rate.

In the Census Bureau’s Director’s Blog, Ron Jarmin wrote the new data “show that distrust in Census Bureau and government may complicate outreach to some communities.” But Jarmin said that the survey results will help inform communications strategies for the 2020 count. He also said that many people will be encouraged to participate because they place a high value on public services whose funding is determined by the census.

Participants in the study’s focus groups raised concerns about the addition of the question. Researchers noted that the citizenship question may be a “major barrier” to participation for some parts of the public, quoting focus-group participants who voiced concerns about “deportation sweeps.” One Spanish-speaking participant said that “it’s like giving the government information, saying ‘Oh, there are more here,’ ” according to slides from the Census Bureau presentation.

At the time of the survey, 53 percent of all respondents correctly said that the decennial census is not used to locate people living in the country without documentation. Ten percent thought the census was used for this purpose, and 37 percent said they were unsure.

“I think the initial evaluation of their research shows that they are facing a significant number of challenges in terms of convincing people that it is both safe and important to participate in the census,” said Terri Ann Lowenthal, a former staff director of the House’s census oversight subcommittee. “The research already shows that the citizenship question could be a new barrier to full participation that the Census Bureau hadn’t contemplated before earlier this year.”