President Obama will detail his immigration plan Thursday night. (Brendan Smialowski/Agence France-Presse via Getty Images)

The political consequences of President Obama’s new immigration plan will probably depend on whether Americans focus on the merits of the policy or on the president’s audacious means of achieving it.

Democrats say that Obama’s action, which would protect millions of illegal immigrants from deportation, is necessary to keep the faith of Latinos, the nation’s fastest-growing minority and a crucial voting bloc for their party in recent elections.

For Republicans, the move represents confirmation that the president is constantly overreaching, with contempt for the concerns of those who disagree with him. At the same time, GOP leaders worry that an overreaction on their own side — shutting down the government over the immigration issue, for instance — could backfire and raise questions about their capacity to govern just as they are poised to take full control of Capitol Hill.

A chorus of Republicans on Wednesday accused Obama of behaving like a monarch — or, as Sen. Jeff Sessions (R-Ala.) put it, “emperor of the United States.” They pointed to a litany of Obama’s own past statements to make their case, noting that he often said there were limits to what he could do in the absence of a comprehensive new immigration law.

While there are decades of precedent for executives — both Democratic and Republican — claiming leeway in choosing which immigration cases to pursue, none has applied “prosecutorial discretion” as broadly as Obama is expected to do.

Obama will argue that his policy is simply an acknowledgment of practical reality: that the government’s limited resources should be devoted to dealing with the cases that pose the greatest public safety threat, and that Congress itself has failed to act.

A comprehensive immigration bill passed the Democratic-led Senate in June 2013, but the GOP-majority House has not moved on the issue.

In the absence of legislation, “the president is somebody who is willing to examine the law, review the law and use every element of that law to make progress for the American people,” White House press secretary Josh Earnest said Wednesday. “If that is something that Republicans are critical of, then that’s . . . a criticism that the president wears with a badge of honor.”

Politically, Obama has little choice: He had promised for months to undertake this kind of executive action, which will be an expansion of his 2012 decision to protect from deportation hundreds of thousands of young people who were brought to the country illegally as children.

“He has boxed himself in completely,” said William Galston, a Brookings Institution senior fellow who was a top domestic policy adviser in Bill Clinton’s White House. “Politically speaking, there has been a promissory note outstanding for a long time.”

Were Obama to fail to follow through, Galston said, he would not only lose credibility, but could damage his party’s relationship with Latinos.

“The road to the White House comes through the [Hispanic] community in critical states,” said Sen. Robert Menendez (D-N.J.). He added that unless Republicans come up with their own proposal for fixing the immigration system, “they’re in trouble.”

The public has been largely supportive of a comprehensive overhaul of the immigration system and sympathetic to the idea of creating a pathway to citizenship for the more than 11 million illegal immigrants who are estimated to be living in the United States.

But a Wall Street Journal/NBC News poll released Wednesday found deep misgivings about Obama’s expected executive action. Nearly half of those surveyed — 48 percent — said they disapprove of the president acting alone on immigration, and only 38 percent said they approved of it.

Former housing secretary Henry Cisneros, a supporter of the president’s action, predicted that opposition will fade. Those who benefit, he said, will be grateful for generations — as he said many Latinos feel toward President Ronald Reagan for the amnesty program he signed into law in 1986.

“While the breadth of anger will be wide, the intensity of the benefits will be deep,” Cisneros said.

Univision, the widely watched Spanish-language channel, has agreed to delay a portion of the 15th annual Latin Grammys in order to carry Obama’s announcement.

Because it is not written into law, the reprieve that Obama grants will by definition be temporary. And Republicans say the action will make it all the more difficult to pass a permanent new law.

“If ‘Emperor Obama’ ignores the American people and announces an amnesty plan that he himself has said over and over again exceeds his constitutional authority, he will cement his legacy of lawlessness and ruin the chances for congressional action on this issue — and many others,” said Michael Steel, a spokesman for House Speaker John A. Boehner (R-Ohio).

Many Republicans noted that Democrats controlled both houses of Congress in the early years of the Obama administration and failed to pass an immigration bill.

“We’ve gone to the president and said, ‘Give us time to do immigration reform, to work on the issue this year. We want to get this done.’ And this is the reaction he has to that?” said Rep. Paul Ryan (R-Wis.), the 2012 vice presidential candidate. “He had two years with a super-majority of his own party, and he didn’t lift a finger. And now he won’t give us a few weeks? He’s basically choosing to give us a partisan bomb.”

But if it detonates, Republicans realize that they could be the ones injured.

“Republicans have to handle this in a sensitive way,” said Sen. Orrin G. Hatch (R-Utah). He warned that any of them who speaks of impeaching the president over immigration would be shunned by most in the party and “considered pretty erratic and pretty excessive.”

Two Republicans who are mulling presidential bids, Sen. Rand Paul (R-Ky.) and Sen. Rob Portman (R-Ohio), cautioned their colleagues to focus on fighting the policy, rather than on fiery rebuttals that could alienate the voters the party must woo in 2016.

“Pass some things,” Paul said, when asked about how the party should react. He suggested an expansion of work visas as an example of one potential piece of legislation.

“No one in our leadership team is interested in shutting down the government,” Portman said. “Many of us believe the immigration system is broken and we’d like to fix it, and we’d like to work with the president on that.”

Robert Costa and Lori Montgomery contributed to this report.