CHICAGO -- The surprise news that longtime immigrant rights advocate U.S. Rep. Luis Gutierrez won't seek re-election after 25 years in office triggered a burst of activity in his Chicago area district, with primary candidates vowing to continue his work.

Three Democrats are walking a fine line on the campaign trail, trying to stand out in the race for Illinois' only open congressional seat while taking pains not to strongly criticize the popular congressman who easily won re-election a dozen times.

Gutierrez is a firebrand credited with helping shape the immigration reform movement, and whose activism and constituent work has rippled beyond Illinois. Questions remain about where a successor in the largely Democratic and Hispanic district new to the national stage will fit, especially as President Donald Trump winds down a program protecting young immigrants from deportation and cracks down on sanctuary cities like Chicago.

The Democratic candidates on the March 20 ballot - narrowed from an initial pool of seven - are a county official with name recognition, a police sergeant and a community activist. Financial analyst Mark Lorch of Riverside is running unopposed in the GOP primary.

Gutierrez shocked political circles in November when he announced he was leaving after 13 terms. Days earlier he filed candidate petitions to run. But he used the news conference to appear side-by-side with his pick to replace him: Cook County Commissioner Jesus "Chuy" Garcia, best known for forcing Mayor Rahm Emanuel into an unprecedented runoff election in 2015.

Both men were protégés of the late Chicago Mayor Harold Washington, with Garcia also serving as a state senator and community organizer. He said he's ready to pick up where Gutierrez left off, with a few changes. Garcia, a Mexican immigrant who bills himself as a "neighborhood guy" is more introverted and reticent. Gutierrez, with family ties to Puerto Rico, has a preacher-like oratory style and a long arrest record for civil disobedience outside the White House and immigration offices.

"We're both very passionate. I may come across as being more subdued, maybe a little quieter because that's kind of my nature," Garcia said. "But in terms of when the fight is on, it's on. That's the upbringing we've had as a Mexican immigrant. Don't let anyone push you around. Don't be a loudmouth. Don't be boastful."

The other candidates say they can compete in the district that has the largest concentration of Latinos of any Midwest congressional district. They make up over 70 percent of the residents, drawing from Mexican, Puerto Rican and Central American neighborhoods of Chicago and its surrounding suburbs. Only a handful of districts in California, Arizona, Texas and Florida have similar demographics.

Immigration has been an issue in the neighborhood, which includes the Midwest's largest Mexican enclave called Little Village. Several raids by federal immigration agents have taken place there over the years, including daytime arrests at a popular shopping plaza in 2007. The area, which is densely populated and has a thriving business district, also has low-income pockets and persistent street crime.

Candidate and community activist Sol Flores, who runs an anti-homelessness organization, said people want an outsider who'll pay attention to new issues. She sidestepped questions about how Gutierrez could have done better, aside from saying she wasn't deterred by his backing for Garcia.

"Voters are smarter than that," she said. "People are excited about something different and something new."

Twenty-five-year Chicago Police Department veteran Richard Gonzalez, who jumped in the race before Gutierrez's announcement said he thought the congressmen could have gone further in the fight for reform, but he first praised his work.

"He is the immigration champion," Gonzalez said. He said his police work in sections of the district gave him an edge on understanding other issues, including affordable housing and education. He vowed to focus on efforts to divert children from crime.

Gutierrez said he has no immediate plans for higher office, but has not ruled out a future run.

He plans to finish his term, travel the country and spend time on hurricane relief efforts in Puerto Rico, where he has a home. He's vaguely mentioned preparations in creating a "new energy" for 2020, but not detailed what that means.

Gutierrez, an early backer of legislation to help immigrants brought to the country without legal permission as children, is a leading member of the Congressional Hispanic Caucus. He's led marches and citizenship town halls nationwide and been critical of Republicans and Democrats for not doing enough, including former President Barack Obama.

Experts credit him with helping convert the push for immigration reform from a "third rail" issue to part of the mainstream discussion.

"Gutierrez is one of a kind," said Frank Sharry, the head of immigration advocacy group America's Voice. "No one will replace him."

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