Rep. Joseph Crowley (D-N.Y.) is hammering the acting head of U.S. Immigration and Customs Enforcement over his plans to speak Tuesday at an event hosted by a hard-line immigration group that many Democrats regard as bigoted.

Crowley, chairman of the House Democratic Caucus, said the scheduled appearance by Thomas Homan, the agency's acting director, lends a dangerous legitimacy to the Center for Immigration Studies (CIS), an anti-immigration think tank that's holding the event at the National Press Club in Washington, D.C.

In a letter sent to Homan on Monday, Crowley characterized CIS as "a hate group" and asked him to cancel his plans to headline the gathering.

"It is highly inappropriate for a senior official of a federal agency to engage with a group that spreads such abhorrent viewpoints, including white supremacism and anti-Semitism," Crowley wrote, "and I urge you to immediately retract your plans to speak."

The entreaty was quickly rebuffed by a spokeswoman for the Department of Homeland Security (DHS), who emphasized that the event - a forum designed to address public safety concerns - is a public event set to be staged at a reputed venue and live-streamed on C-SPAN.

"This [is] a valuable opportunity for productive civil discourse and a healthy debate on a topic of incredible interest," Katie Waldman said Monday night in an email.

"DHS is committed [to] sharing with the public facts about immigration and border security. We expect it will be informative for the public - even members of Congress."

Crowley's charges against CIS lean heavily on the analyses of the Southern Poverty Law Center (SPLC), a liberal activist group that tracks extremist activities around the country.

In 2016, the organization put CIS on its annual list of "hate groups," where it's remained since. SPLC cited CIS's "repeated circulation of white nationalist and antisemitic writers in its weekly newsletter" and its publication of writings by Jason Richwine, a former analyst at the Heritage Foundation who resigned in 2013 following a public airing of his college dissertation, which asserted that immigrants have lower IQs than "the white native population."

CIS, which advocates both for lower levels of legal immigration and tougher enforcement against those in the country illegally, has rejected all charges of bigotry. Writing in The Washington Post last year, CIS Executive Director Mark Krikorian said SPLC's hate-group listing was merely "an attempt to delegitimize and suppress views regarding immigration held by a large share of the American public."

"The wickedness of the SPLC's blacklist lies in the fact that it conflates groups that really do preach hatred, such as the Ku Klux Klan and Nation of Islam, with ones that simply do not share the SPLC's political preferences," he wrote.

Krikorian acknowledged that, on occasion, the CIS newsletter has referenced authors "who turned out to be cranks."

The debate over immigration - and CIS's role in it - has exploded under President Trump, whose ascension to the White House featured a central promise to crack down on both illegal immigration and those already living in the country illegally.

Homan, a former Border Patrol agent with more than three decades in law enforcement, is on the front lines of that fight. And he's largely adopted the hard-line positions of Trump, warning immigrants "to be worried" about tougher enforcement and advocating for criminal charges against politicians who defend so-called sanctuary cities.

Most recently, the administration has raised concerns among immigrant rights advocates for a policy shift that separates immigrant families attempting to cross the border illegally, even when they're first-time offenders.

The moves have infuriated Democrats on Capitol Hill, who contend the separations are harming innocent children without addressing the root cause of the migrations, which are frequently undertaken to flee violence in Central America.

Crowley is among the most vocal critics of the policy shift and made a point to press his case to Homan, who's retiring later this month.

"Lending legitimacy to a hate group and splitting children from families does not in any way protect national security and public safety, nor does it demonstrate the integrity and fairness that is expected of a federal agency. Instead, it fosters an environment in which extremist viewpoints come to be seen as acceptable and justified by the highest levels of your agency," Crowley wrote.

"Speaking to a group like this is disqualifying for a federal official entrusted with acting in the best interests of the general public."

It's not the first time CIS has been in the spotlight recently. Last month, Trump nominated a CIS fellow, Ronald Mortensen, to head the Bureau of Population, Refugees and Migration under the State Department. Mortensen, a former foreign service officer, has come under fire from immigration advocates for his tough approach toward "Dreamers," young immigrants who came to the country illegally as kids, and his path to confirmation in the Senate is uncertain.

At least one Republican, Sen. Jeff Flake (Ariz.), an immigration reform advocate who is retiring after this year, is opposing Mortensen's nomination.