During his speech on immigration reform, President Obama called on illegal immigrants to "come out of the shadows" and "get right with the law." (AP)

President Obama sought to convince the American public Thursday that his plans to unilaterally change immigration laws were well within the precedent set by previous administrations and did not amount to an amnesty program for illegal immigrants.

In a prime-time address from the White House, Obama argued that a mass deportation of the nation’s more than 11 million undocumented immigrants “would be both impossible and contrary to our character.”

Rather, the president said, the measures he is enacting to defer the deportations of 4 million immigrants while simultaneously refocusing federal border control agents on the highest-priority cases, such as felons, gang members and recent border-crossers, are aimed at “actual threats to our security.”

“Felons, not families,” Obama said of who would be in line for deportations. “Criminals, not children. Gang members, not a mom who’s working hard to provide for her kids.”

Under Obama’s plan, the undocumented parents of U.S. citizens and legal permanent residents who have lived in the country for at least five years can apply, starting this spring, for relief from deportations for a period of three years. About 3.7 million immigrants are expected to qualify under the new guidelines.

President Obama will take executive action Thursday to offer temporary legal status to millions of undocumented immigrants.

The president also is expanding a 2012 program that has provided administrative relief to nearly 600,000 young people brought to the country illegally as children. Officials said that expansion, which will remove an age cap, could reach another 287,000 people.

The president’s speech, which lasted 15 minutes, was an effort to build public support and head off staunch opposition from congressional Republicans who have vowed to fight Obama’s use of executive actions to circumvent the legisative branch after an effort to pass a comprehensive immigration bill failed on Capitol Hill last summer.

Even before Obama took to the airwaves, GOP leaders were deliberating over their next move over how to stop him. Republicans in both chambers debated filing a lawsuit over the president’s use of executive authority, pursuing their own legislation on immigration policy or removing funding for federal immigration agencies.

“We are considering a variety of options,” Sen. Mitch McConnell (R-Ky.), who will become majority leader next month, said Thursday in a floor speech. He suggested that his preference would be for Republicans to avoid becoming mired in a fiscal clash during the lame-duck session, shortly before the GOP takes control of the Senate.

But White House lawyers expressed confidence that Obama has the legal standing to enact the changes. They cited previous executive actions taken by Republican presidents, including Ronald Reagan and George H.W. Bush, both of whom signed orders protecting smaller groups of illegal immigrants from deportation.

In his remarks, Obama quoted another Republican — his predecessor, George W. Bush-- while making the case that illegal immigrants “are a part of American life.”

Asked about a potential GOP lawsuit, a senior administration official said: “Anyone with a filing fee can sue, there’s nothing we can do about that.” But the official, who spoke on condition of anonymity to describe the president’s thinking, added that administration lawyers believe Obama’s actions “are absolutely supported by the law.”

Obama portrayed his approach as a “commonsense, middle-ground approach” that will allow otherwise law-abiding immigrants to “come out of the shadows and get right with the law.”

Addressing the chief criticism of Republicans — that illegal immigrants are being rewarded for violating the law to remain in the country — Obama declared that his policy is not amnesty.

“Amnesty is the immigration system we have today – millions of people who live here without paying their taxes or playing by the rules, while politicians use the issue to scare people and whip up votes at election time,” Obama said. “That’s the real amnesty – leaving this broken system the way it is.”

The plan to offer immigrants who qualify for Obama’s executive action plan three years of relief would mean that the newly processed applicants would be protected from deportations through the first year of Obama’s successor in 2017. That would leave it up to the new administration to determine whether to continue the program or abruptly eliminate it.

The new protections are a year longer than under the 2012 program for younger immigrants, known as Deferred Action for Childhood Arrivals, and officials said that the DACA program also would be revised to provide three years of relief. It also would change the date by which applicants must have arrived in the United States from June 15, 2007, to Jan. 1, 2010, officials said.

Administration officials said the president’s decision to protect illegal immigrant parents of U.S. citizens and permanent residents from deportation is based a “humanitarian” rationale that recognizes special legal statutes Congress has enacted to keep together families, administration officials said Thursday.

Under current law, immigrant children living legally in the United States are permitted to apply for visas for their parents, and Obama’s decision to defer the deportations of up to 4 million undocumented immigrants is consistent with that precedent, the officials said.

“The impact of the contrary position would be that mothers and fathers and their children could be separated,” said one senior administration official, who spoke on condition of anonymity in a briefing for reporters ahead of Obama’s prime-time address. “Congress has recognized this is a particular relationship that Congress is willing to protect.”

Obama alluded to the skilled worker piece of his plan at the White House on Thursday morning, during a ceremony to honor top American scientists.

“Part of staying competitive in a global economy is making sure that we have an immigration system that doesn’t send away top talent, but attracts it,” Obama said to applause, according to a transcript of the event.

If Obama follows through, his plan would be the most significant alteration in the U.S. immigration system since the 1980s.

Obama believes he can make the changes based on his own executive authority: White House officials said they spent months making sure that the president would do only what the law allows. Obama is acting after Congress — divided between a Republican-controlled House and a Democratic majority in the Senate — was unable to agree on an immigration overhaul.

But, even before Obama’s changes were announced, Republicans in Congress denounced them as reaching far beyond his constitutional powers.

A spokesman for House Speaker John A. Boehner (R-Ohio), for instance, referred to the president as “Emperor Obama” and charged that he was exceeding his constitutional authority and cementing “his legacy of lawlessness.”

“This is presidential overreach of monumental proportions,” said Sen. Jeff Sessions (R-Ala.), who is likely to be the next chairman of the Senate Budget Committee. “It’s a direct erosion of congressional power. . . . I think Congress needs to and will resist and do so in a way that is responsible.”

But now, Republican leaders in Congress face a tricky question.

What, exactly, should they do about it?

The party’s leaders have advocated tactics that — they say — would provide a forceful response, without shutting down the government itself. They might file a lawsuit, challenging Obama’s authority in court. They might craft legislation to make their own revisions to immigration policy.

But many in the Republican rank-and-file believe those tactics would not be forceful enough.

Instead, Sen. Ted Cruz (R-Tex.) said on Wednesday that Obama’s executive action should be met with a refusal to vote on any more of his nominees. On Thursday, looking for a parallel to Obama’s actions, Cruz reached back to ancient Rome, comparing Obama to the Catiline conspiracy, a plot to overthrow the Roman Republic.

Sessions has advocated a series of short-term spending bills. The idea would be to create a series of deadlines: if Congress did not pass a new spending bill, the government might shut down. Sessions believes that these deadlines might give Republicans leverage to demand concessions from Obama on immigration.

But the risk for Republicans is that, if Obama does not cave the way Sessions expects, the government might actually shut down, and Republicans might get some of the blame.

Rep. Steve King (R-Iowa) has hinted at another possibility: bringing up articles of impeachment against Obama.

More moderate Republicans worry that these actions could alienate Latino voters and repeat the self-inflicted wounds of the 2013 government shutdown.

“The president wants to see an angry and intemperate response, thinking the Republicans will do something that leads to a shutdown,” said Rep. Charlie Dent (R-Pa.). “Don’t take the bait, and don’t have a hysterical reaction. We can be strong, rational and measured.”

On Wednesday, Obama discussed his plans with more than a dozen congressional Democrats during a dinner at the White House, outlining why he believes his actions are “the first big step” in reforming the nation’s immigration system. But Obama also emphasized that his actions are not going to be a permanent solution.

Rep. Judy Chu (D-Calif.), who chairs the Congressional Asian Pacific American Caucus and attended Wednesday night’s dinner, said the president “was very clear on the fact that we have to do this now and remain strong through this process.”

The showdown with Republicans comes two years after Obama, flush off his 2012 reelection, announced he would pursue a legislative overhaul of immigration laws as he sought to fulfill a campaign promise to Latino and Asian American supporters who were frustrated that the president had not done more on immigration in his first term.

But an 18-month effort to pass legislation, which included a path to citizenship, collapsed this summer in the face of strenuous opposition in the Republican-controlled House. In June, Obama promised to act aggressively on the problem in areas where the law allowed the use of his executive authority.

Administration lawyers have spent months reviewing case law and meeting with immigration advocates, law enforcement officials and legal experts to develop options for Obama, who reviewed them with Department of Homeland Security Secretary Jeh Johnson and Attorney General Eric H. Holder Jr.

In addition to his plan to protect some immigrant parents from deportation, Obama is also expected to expand a 2012 program that has deferred the deportations of nearly 600,000 younger immigrants known as “Dreamers,” who were brought into the country illegally as children. (The name comes from the Dream Act, proposed legislation that Congress has failed to pass.) Obama’s plans would expand that program by raising the maximum current age from 30 and raising the maximum arrival age above 16. However, it is not known how many years of eligibility he will add at either end or how many more people will be covered.

“I am getting slammed with calls from people asking me if they will qualify, but I can’t tell them anything yet,” said Simon Sandoval Moshenberg of the Legal Aid Justice Center in Falls Church, Va., which has helped scores of young immigrants apply for the deferred-action program over the past two years. “Any time you draw a line, there are people who fall on either side. It was heartbreaking to see people miss the age cutoff date by just a few weeks, and that can happen again.”

The White House, recognizing the political risks, moved to shore up public support with an aggressive sales job. Obama released a video on Facebook, explaining his reasons for taking action. By Thursday afternoon, the video had been watched more than 3.3 million times.

His prime-time address Thursday also is aimed at including “as many people across the country in this broader debate” as possible, White House press secretary Josh Earnest said. However, most of the major television networks announced they will not carry the remarks live, leaving it to the cable news channels and the Spanish-language network Univision, where Obama’s remarks will come right before the airing of the Latin Grammy Awards.

On Friday, Obama will return to Las Vegas’s Del Sol High School, which holds a special significance for him. On Jan. 11, 2008, then-Sen. Obama arrived in that city after losing to then-Sen. Hillary Rodham Clinton in New Hampshire’s presidential primary. Obama pivoted to the heavily Latino voting bloc of Nevadans and delivered what became known as his “Si Se Puede” speech — the Spanish equivalent of “Yes We Can.”

At the high school that day, hundreds filled the gymnasium, and hundreds more waited outside as the president vowed to deliver on citizenship for undocumented immigrants.