LAS VEGAS--President Obama kicked off his new immigration plan in Las Vegas on Friday, including signing an executive action offering a reprieve from deportation to millions of immigrants..

The president knew he would find supporters there. Undocumented workers make up about 10% of Nevada's labor force, more than in any other state. Illegal immigrants work up in casinos and restaurants and down the Las Vegas Strip. The Culinary Workers Union, which represents 55,000 workers, was a key backer of Mr. Obama's re-election bid in 2012. Nevada's Latino voters chose him over Republican Mitt Romney in the 2012 election by 71% to 27%.

"The message to Nevada's Latinos is not lost on going to Las Vegas," said Louis DeSipio, professor of political science at the University of California, Irvine, and an expert on Hispanics. "Nevada is now a purple state precisely because of Latinos. It's important for the president and the party, starting with [Senate Majority Leader] Harry Reid."

Democrats are working to energize Latinos ahead of the 2016 election, when Mr. Reid (D., Nev.)--who will cede his position to Sen. Mitch McConnell (R., Ky.) in January--is expected to run for re-election. Mr. Reid has said he will attend the Friday rally.

About one-third of Nevada's population is Hispanic. Because of its service-based economy, Nevada has been an engine for Latino success, said Mr. DeSipio. Immigrant families, in particular, have flocked here in large numbers. Many of them are mixed-status, illegal workers with U.S.-born children.

In a state where nearly 8% of residents are in the U.S. illegally, one in every six children has an undocumented parent. This year, Nevada, which has a Latino Republican governor, joined about a dozen other states that allow undocumented immigrants to legally drive by obtaining special authorization cards.

The Friday event took place at a Las Vegas high school, Del Sol, where almost two-thirds of the students are Hispanic.

The visit marked the president's third to the high school. He first came here during his 2008 presidential campaign. In January 2013, he chose the same high school to push for a broad plan for an immigration overhaul to legalize some 11 million immigrants. A comprehensive bill passed the Senate but died in the GOP-controlled House.

Under Mr. Obama's plan, people who have been in the U.S. for at least five years and are parents of citizens or legal permanent residents will be eligible to apply. Nearly a million more could benefit through other new or expanded programs. The president has also narrowed the group of people who will be subject to deportation, with the goal of focusing enforcement resources on gang members, serious criminals and those who crossed the border after Jan. 1, 2014.

Republicans criticized the move as presidential overreach, and are considering possible ways of blocking its implementation.

"The president began this conversation in Vegas and wants to make the point that he has been trying and he is back with something, and it's Congress that has been the obstructionist," Mr. DiSipio said.

Among those likely to benefit from the sweeping changes is the family of Astrid Silva. Ms Silva, 26, received a deferral of deportation and a work permit thanks to a program Mr. Obama unveiled in 2012 for young immigrants brought to the country illegally as children. That program, Deferred Action for Childhood Arrivals, or DACA, was the first major unilateral action taken by the president.

In his White House speech Thursday evening, the president held up Ms. Silva as a model immigrant, working on her third degree. When she crossed the border from Mexico at age 4, "her only possessions were a cross, her doll and the frilly dress she had on. When she started school, she didn't speak any English. She caught up to other kids by reading newspapers and watching PBS, and she became a good student," he said.

In an interview Friday, Ms. Silva said she is hopeful that her mother, a cleaning lady, and her father, who works in landscaping, will be eligible for the new program because her brother, Cesar, was born in the U.S.

Immigrant organizations from several states have sent busloads of supporters to Las Vegas to attend the event at the high school. Among them is the Coalition for Humane Immigrant Rights of Los Angeles, which is planning to hold an event at the Los Angeles Convention Center next month to teach immigrants about the new policies.

Protesters marched at the Las Vegas airport and outside the high school where Mr. Obama spoke. Early Friday, a handful of demonstrators stood outside the high school holding signs that read, "Stop Obama," "Obama Lies" and "Help Our Homeless."

Meanwhile, Ms. Silva, in a purple dress, high heels and with her hair curled, said she was ready to meet the president and even more eager to see her family reap the rewards of the new program.

For fear of deportation, "we were never sure if this would be our last Christmas all together," said Ms. Silva, co-founder of Dream Big Vegas, an undocumented-student group.

(Correction: President Barack Obama chose Las Vegas as the place to sign his presidential memorandum, an executive action. An earlier version of this article incorrectly stated it was an executive order.)

Write to Miriam Jordan at miriam.jordan@wsj.com

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