Four in 10 American voters think there is “cause to wonder” whether Barack Obama was born in the United States -- and nearly one in four believes outright that he wasn’t.

Only “natural born citizens” are eligible to be president under the U.S. Constitution.

A Fox News poll released Thursday found 24 percent of voters believe President Obama was born somewhere other than the United States.

As one might expect, there are huge partisan differences on this issue. Still, 12 percent of Democrats think the president was born elsewhere, as do 21 percent of independents. That percentage climbs to 37 percent among Republicans. Among those who consider themselves part of the Tea Party movement, 41 percent think the president was born outside the U.S.

Most American voters -- 67 percent -- believe Obama was born in the United States. That includes almost all Democrats (84 percent) and most independents (69 percent). Less than half of Republicans (47 percent) and Tea Partiers (44 percent) think so.

Click here for complete poll results.

Questions about Barack Obama’s birthplace began during the 2008 presidential campaign -- despite birth announcements from newspaper archives and the state of Hawaii releasing birth records. The issue was recently re-ignited by possible Republican presidential candidate and businessman Donald Trump, who on several television talk shows said the president should produce an original “long-form” birth certificate.

So far this line of questioning hasn’t helped Trump garner an advantage over Obama. In a hypothetical 2012 match-up, if the election were held today, 34 percent say they would vote for Trump if he were the Republican candidate and 52 percent say they would back President Obama.

How do voters view those people -- sometimes referred to as “birthers” -- who continue to question publicly whether Obama was born in this country? Just over half -- 51 percent -- think they are “nutty conspiracy theorists.”

A large 40-percent minority, however, thinks there is “cause to wonder” where the president was born and that this justifies the actions of birthers.

Most Democrats (74 percent) and almost half of independents (48 percent) think birthers are “nutty conspiracy theorists.” So do 29 percent of Republicans and 24 percent of Tea Partiers.

The flip side is that 72 percent of Tea Partiers and 63 percent of Republicans think there is cause to wonder about the president’s birthplace. Some 41 percent of independents and 20 percent of Democrats agree.

Even 21 percent of those who say they believe Obama was born in the U.S. think there’s reason to wonder if they’re wrong.

The Fox News poll is based on landline and cell phone interviews with 914 randomly chosen registered voters nationwide and was conducted under the joint direction of Anderson Robbins Research (D) and Shaw & Company Research (R) from April 3 to April 5. For the total sample, it has a margin of sampling error of plus or minus 3 percentage points.