The political backlash against U.S. Immigration and Customs Enforcement (ICE) has turned so intense that leaders of the agency’s criminal investigative division sent a letter last week to Homeland Security Secretary Kirstjen Nielsen urging an organizational split.

The letter, signed by the majority of the special agents in charge of ICE’s Homeland Security Investigative Division (HSI), offered a window into growing internal tension at the agency as an “Abolish ICE” protest movement has targeted its offices and won support from left-wing Democrats.

Though ICE is primarily known for immigration enforcement, the agency has two distinct divisions: Enforcement and Removal Operations (ERO), which carries out immigration arrests and deportations, and HSI, the transnational investigative branch with a broad focus on counterterrorism, narcotics enforcement, human trafficking and other crimes.

The letter signed by 19 special agents in charge urges Nielsen to split HSI from ICE, because anger at ERO immigration practices is harming the entire agency’s reputation and undermining other law enforcement agencies’ willingness to cooperate, the agents told Nielsen.

Since President Trump’s inauguration, the state of California and several of the country’s largest cities have barred their law enforcement agents from cooperating with ICE by declaring themselves “sanctuary” jurisdictions. That has made it increasingly difficult for HSI agents to fight drug cartels and conduct major criminal investigations in the country’s largest urban areas, the letter said.

“The perception of HSI’s investigative independence is unnecessarily impacted by the political nature of ERO’s civil immigration enforcement,” the agents told Nielsen.

Trump took office promising to quickly deport “two or three million” foreigners, and following his inauguration, ICE interior arrests jumped nearly 40 percent. In recent months the agency has resumed carrying out large-scale workplace raids, winning glowing praise from the president, who said at a rally in North Dakota on Wednesday that ICE agents are “mean but have heart,” and that they are “liberating” U.S. communities from the MS-13 gang.

Trump officials say they fear the transnational gang, whose members the president calls “animals,” could take advantage of lax enforcement at the border.

In their letter to Nielsen, the agency’s top investigators painted a starkly different picture — telling her their crime-fighting capability is being stifled by the political heat.

“Many jurisdictions continue to refuse to work with HSI because of a perceived linkage to the politics of civil immigration,” the investigators wrote. “Other jurisdictions agree to partner with HSI as long as the ‘ICE’ name is excluded from any public facing information.”

In one indication of eroding morale, the special agents told Nielsen that making HSI its own independent agency, “will allow employees to develop a strong agency pride.”

The letter, marked “Law Enforcement Sensitive,” was first reported by The Texas Observer.

ICE’s acting director, Thomas D. Homan, has been a vocal Trump supporter and an unabashed enthusiast of the president’s immigration agenda. But he has announced his retirement and is stepping down this month, and a nominee to replace him has yet to be named.

Nielsen has not publicly responded to the letter.

A senior ICE official in Washington said the HSI agents’ letter was “not well received” at the agency’s headquarters in Washington, calling it “ill conceived and poorly timed” at a moment when so many staffers feel besieged by the backlash.

he proposal to reorganize ICE is not a new one, the official said, but “has never been taken seriously” and would “require congressional action.”

The senior official spoke on the condition of anonymity to candidly discuss reaction to the letter within ICE’s top ranks.

The official conceded that the special agents’ arguments have “some merits,” adding, “the concerns they raise in the letter are certainly operational obstacles and worthy of discussion.

But the official called the notion of breaking up ICE “a non-starter” and said it was inappropriate for the agents to go outside established internal channels to take their gripes directly to Nielsen in a letter that quickly leaked to reporters.

“Our employees are being protested, threatened and unfairly attacked,” the senior official said.

The Abolish ICE movement has gained new momentum in recent weeks amid public outcry over the Trump administration’s “zero tolerance” crackdown that separated more than 2,500 migrant children from their parents along the Mexico border. A federal judge this week ordered the government to reunite them with their parents — many of whom are currently held in ICE custody — within 30 days.

In New York City this week, Democratic congressional candidate Alexandria Ocampo-Cortez won an upset primary victory Tuesday after taking up the Abolish ICE cause as one of her campaign promises, while in Portland, Ore., protesters have set up a sprawling tent camp outside ICE’s local office.

Devlin Barrett contributed to this report.