Half of self-identified Democratic primary voters in a new INSIDER poll supported turning improper entry to the United States into a civil offense rather than a criminal offense, which would effectively decriminalize illegal immigration.

The results reflect how the party's progressive base could push the eventual nominee further to the left on immigration.

The portion of federal law that makes improper entry a criminal offense has generated controversy over its use in the Trump administration's hardline "zero tolerance" immigration policies.

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Half of self-identified Democratic primary voters in a new INSIDER poll supported making improper entry into the United States a civil offense rather than a criminal offense, which would effectively decriminalize illegal immigration.

We asked more than 1,100 respondents in a poll conducted through SurveyMonkey Audience whether they would support or oppose making improper entry a civil offense, rather than a criminal misdemeanor.

Among 460 respondents who said they'd participate in their state's Democratic primary or caucus, half supported or strongly supported making improper entry a civil offense. Twenty-three percent neither supported nor opposed it, and 18% opposed or strongly opposed it. Eight percent said, "I don't know."

Read more: Julián Castro earned rave reviews for his debate performance, but it was even better than it looked

Within the wider pool of 1,110 adult respondents, 37% indicated they would support or strongly support making improper entry a civil offense, 22% neither supported nor opposed it, and 29% opposed or strongly supported it. Twelve percent responded, "I don't know."

The results reflect how the party's progressive base could push the eventual nominee further to the left on immigration. While most candidates oppose President Donald Trump's border wall on the US-Mexico border and the travel ban, the primary field has been thin on specific plans outlining how they would tackle key immigration issues — among them, how they would chart a path to citizenship for the country's unauthorized immigrants, a popular position among Democratic voters.

Former Housing and Urban Department Secretary Julián Castro was the first candidate in the primary field to propose repealing a portion of federal law, known as Section 1325, that makes crossing into the United States a criminal offense rather than a civil offense. It means migrants crossing the border illegally could be charged with a criminal misdemeanor and then jailed, fined, or deported.

Castro sparred over the issue with former Rep. Beto O'Rourke of Texas on the first night of the Democratic primary debates last week. He called out O'Rourke, saying, "Some of us on this stage have called to end that section, to terminate it, some, like Congressman O'Rourke, have not."

Read more: An obscure section of US law became a flashpoint in the Democratic debate and sparked a confrontation between Julián Castro and Beto O'Rourke. Here's why it's so controversial.

A part of federal law dating back to 1929, Section 1325 has generated controversy over its use in the Trump administration's hardline "zero tolerance" immigration policies. The administration used it to justify charging all migrants crossing the border with a crime, which led to migrant parents to be separated from their children and triggered a large outcry over their poor treatment.

SurveyMonkey Audience polls from a national sample balanced by census data of age and gender. Respondents are incentivized to complete surveys through charitable contributions. Generally speaking, digital polling tends to skew toward people with access to the internet. SurveyMonkey Audience doesn't try to weight its sample based on race or income. The poll was collected June 28-29, had a total of 1,172 respondents, and a margin of error of plus or minus 3.02 percentage points with a 95% confidence level.