After months of negotiations failed to reach a settlement over the allegations, the U.S. Justice Department took the rare step Thursday of suing.

“We have invariably been able to work collaboratively with law enforcement agencies to build better departments and safer communities,” Assistant U.S. Attorney General Thomas Perez said.

Arpaio and his department “have been a glaring exception,” said Perez, who heads the civil rights division.

The main issue that caused talks to break down last month was federal officials’ insistence that Arpaio agree to a court-appointed monitor for the department. Arpaio objected, saying it would undermine his authority.

“I am not going to surrender my office to the federal government,” a visibly mad Arpaio said at an afternoon news conference. “I will fight this to the bitter end.”

The lawsuit means that a federal judge will choose the escalating, long-standing dispute.

The Justice Department, which had been investigating Arpaio on civil rights allegations for more than three years and faced a similar impasse earlier in the investigation, said it was left with no choice but to sue the sheriff to seek the court-appointed monitor it wants to oversee the law enforcement agency.

The DOJ had filed another lawsuit against Arpaio that alleged his office refused to fully cooperate with a request for records and access to jails and employees. It was settled last summer after the office complied.

The latest lawsuit comes as part of the DOJ’s effort to enforce a law passed after the verdict in the Rodney King police brutality case and the Los Angeles riots. It bans police from systematically violating constitutional rights.

Normally, settlements are filed in court as part of lawsuits that aren’t contested by the police agencies.

Since the law’s passage, federal officials said that only once before has the Justice Department filed a lawsuit against a police department with which they were unable to reach an agreement.

In 1999, they filed a lawsuit against Columbus, Ohio, police, but the two sides eventually settled, Perez said.

The DOJ first leveled the allegations against Arpaio in December, saying a culture of disregard for basic constitutional rights prevailed at his office, which covers the Phoenix metropolitan area.

Arpaio’s office is accused of punishing Hispanic jail inmates for speaking Spanish and launching some patrols based on complaints that never reported a crime but conveyed concerns about dark-skinned people congregating or speaking Spanish.

The DOJ has been trying to require Arpaio’s office to train officers in how to make constitutional traffic stops, collect data on people arrested in traffic stops and assure Latinos that the department is there to protect them.

One of the examples cited in the lawsuit was a Latino woman who is a U.S. citizen and was five months pregnant when she was stopped as she pulled into her driveway.

When the woman refused to sit on the hood of a car as the officer insisted, the deputy pulled her arms behind her back, slammed her stomach-first into the vehicle three times and dragged her to his patrol car, the lawsuit said.

He shoved her into the back seat and made her wait about 30 minutes without air conditioning.

Eventually, the woman was cited for failure to provide proof of insurance, but the matter was resolved when she gave such proof to a court, the lawsuit said.

Among the other allegations was that sheriff’s supervisors used county accounts to send emails that demeaned Latinos, such as one that had a photo of a mock driver’s license for a fictional state called “Mexifornia.”

The lawsuit also said Arpaio and his office filed criminal cases and lawsuits against critics. “Nobody is above the law, and nobody can misuse the legal process to silence those with different opinions,” Perez said.

The lawsuit mentioned hundreds of sex-crime cases that Arpaio’s office either failed to adequately investigate or didn’t investigate at all over a three-year period ending in 2007. The sheriff’s office said the backlog was cleared up after the problem was brought to Arpaio’s attention.

“Faced with such an increase in crime and the risk of harm presented by unaddressed sexual assaults, a law enforcement agency ordinarily would be expected to prioritize more serious offenses, such as crimes of sexual violence, over less serious offenses, such as low-level immigration offenses,” the lawsuit said.

Arpaio said the Obama administration brought the lawsuit as a way to court Latino voters in a presidential election year. “They want to send a message that they are taking on the sheriff,” Arpaio said.

Justice Department officials said its investigation of Arpaio’s office took longer than they wanted because Arpaio refused to provide records and access to his jails for 18 months.

The sheriff has demanded that the Justice Department provide facts to prove its allegations. Federal officials have said a 22-page letter sent to Arpaio in December provided those details.

Arpaio is a national political fixture who built his reputation on jailing inmates in tents and dressing them in pink underwear, selling himself to voters as unceasingly tough on crime.

Along the way, he aggressively pushed for a stronger role for local police to enforce immigration law, launching 20 patrols looking for illegal immigrants since January 2008.

During the patrols, deputies flood an area of a city — in some cases, heavily Latino areas — over several days to seek out traffic violators and arrest other suspected offenders. Over the last three years, he raided 58 businesses suspected of breaking a state law by knowingly hiring illegal immigrants.

Lydia Guzman, leader of the civil rights group Respect-Respeto, said the federal lawsuit will go a long way toward convincing the public that Arpaio is abusing his power.

“The reason he has been digging in his heels is because he doesn’t want this to come out,” she said. “Once we’re in trial and it all comes out, it will be an opportunity for all of this to be exposed. The voters will see it.”