People hold a sign during a protest called “Wall Off Trump” near the Quicken Loans Center, site of the Republican National Convention, in Cleveland. (Justin Lane/EPA)

Donald Trump will deliver a high-stakes immigration speech Wednesday in Phoenix amid intense confusion over the status of his vow to deport millions of undocumented immigrants, a potentially damaging impasse for a Republican nominee who risks alienating several voting blocs he needs to win in November.

The immigration speech will come hours after Trump plans to travel to Mexico to meet with that country’s president, Enrique Peña Nieto. The trip was announced late Tuesday.

The campaign has spent days dismissing questions about whether Trump’s positions on immigration are shifting, instead emphasizing his commitment to securing the border with Mexico. But the scale and scope of potential deportations remain subjects of intense interest — and Trump is caught between appeasing his staunchest supporters or attempting to appeal to moderate Republicans and independent voters with a softer stance.

Questions about what would become of the estimated 11 million undocumented immigrants if Trump is elected president have gone largely unanswered by the candidate and his team in recent days. The campaign has suggested that Wednesday’s speech will address those questions and concerns.

Republican presidential candidate Donald Trump has said over and over he would force undocumented immigrants to leave the country as president. Now a meeting with a Hispanic advisory panel and statements from his surrogates are calling into question whether that's still the plan. (Sarah Parnass/The Washington Post)

“I expect the speech to be a refinement of the goals he’s always stated,” said Rep. Kevin Cramer (R-N.D.), a Trump supporter. Cramer said that he would like to see Trump lay out a “chronology” of actions that he would try to achieve as president. The congressman said he is open to a plan that would afford illegal immigrants who have not committed other crimes some form of legal status.

At a campaign stop in Dalton, Ga., on Tuesday, running mate Mike Pence billed Trump’s speech as a chance for the nominee to get highly specific about his plans. Responding to a question about illegal immigration, Pence advised, “Wait about 24 hours, you’re going to hear a lot of details.”

“Don’t miss it. Okay? It’s going to be a very important address,” he added later.

Trump has offered glimpses of his policy priorities even as he has skirted questions about their implementation. He remains publicly committed to building a massive wall along the U.S.-Mexico border, has extolled the need to crack down on those who overstay their visas and has proposed expanding the E-Verify program, used by employers to determine whether an immigrant is legally able to work in the country. The campaign has also said that Trump would prioritize the deportation of criminals, a policy that the Obama administration has also pursued.

Even with those broad priorities in place, it is unclear how many people would be subject to immediate deportation — all undocumented immigrants, all who have overstayed their visas or just those who have committed serious crimes.

The answers to those questions would define the mandate placed on security agencies such as Immigration and Customs Enforcement.

The Rio Grande winds along the U.S.-Mexico border this month near McAllen, Texas. Donald Trump has vowed to build a massive wall along the border and force Mexico to pay for it. (John Moore/Getty Images)

According to a study by the Migration Policy Institute, an estimated 690,000 undocumented immigrants have significant criminal histories — felony convictions or serious misdemeanors — that make them top priorities for deportation under current administration policy. The number of individuals prioritized for deportation would grow to about 5.5 million if visa overstays were included, according to some data, although those estimates are not considered very reliable.

Immigration reform has been at the center of Trump’s brand since he announced his campaign, which he kicked off by railing against Mexican immigrants, whom he suggested were largely criminals and rapists. His controversial rhetoric delighted supporters and enraged critics, who accused him of bigotry. As the Republican primary contests unfolded, he vowed to kick out all of the nation’s illegal immigrants and called for a “deportation force” to get the job done.

But since securing the GOP presidential nomination this spring, Trump has had trouble shifting to an effective general-election strategy after dominating the primary race. And he has struggled to prove that he has a sufficient grasp of policy issues to implement his proposals if elected, a weakness that his Democratic rival, Hillary Clinton, has repeatedly sought to exploit.

His opaque responses to questions about the implementation of those plans — and about mass deportations in particular — have raised concerns among his most vocal supporters while fueling accusations of hypocrisy from his critics.

The candidate has received a deluge of conflicting advice and perspectives from advisers and supporters.

Former Texas governor Rick Perry said in an interview Tuesday that immigration policy — and specifically what to do with the millions of undocumented immigrants in the United States — has confounded public officials for years and that Trump’s critics should be patient and appreciate the nuances of the issue.

“I think those that try to just get this completely black and white at this particular point in time are chasing a rabbit that’s hard to catch,” Perry said.

Perry, who was governor of the nation’s second-most-populous border state for 14 years, said that he and former New York mayor Rudolph W. Giuliani spent time with Trump last week in Texas and had a long discussion about how to treat undocumented immigrants who have lived in the country for years without incident.

“People who’ve been here for a long period of time, who’ve lived within our laws, who’ve paid our taxes, there is a thoughtful way for those individuals to be here legally, and Donald Trump, I am very comfortable, will find solutions to this issue that has flummoxed Washington and the political class for literally decades,” Perry said.

Mark Krikorian, a hard-line opponent of illegal immigration, said in an interview that he has been troubled by Trump’s recent language because it has echoed viewpoints championed by reform advocates such as former Florida governor Jeb Bush and Sen. Marco Rubio (R-Fla.).

“What really set me off was that not only was he musing publicly about what an amnesty would look like, it was that he was using the terminology of the so-called comprehensive immigration reform crowd.”

In one example cited by Krikorian, Trump raised the possibility of making some illegal immigrants pay back taxes as a condition of staying in the country, an idea championed by pro-reform Republicans. Krikorian said the national debate about immigration should focus completely on enforcing laws and securing the border to prevent more illegal immigrants from entering in the future.

“Anything that seems to suggest the kind of guarantee of an amnesty would, I think, as a policy matter be a bad idea — and as a political matter it would be a bad idea,” he said.

Rep. Chris Collins (R-N.Y.), Trump’s first congressional endorser, said in an interview that he is hopeful Trump will lay out a position that allows illegal immigrants who have otherwise followed the law to stay in the country legally.

“That’s what my dairy and crop farmers are asking for,” Collins said, adding: “We were never going to put all these folks — have them come out of the shadows, willingly get on a bus, go back to Mexico and hope to get back.”

He said he also expects Trump to lay out his proposals for border security, including building a physical wall. Collins said he was corrected after he suggested recently that Trump may end up building a “virtual wall.”

“I got my hand a slapped on the wall, and so I’m never going to say that again,” Collins said.

Trump’s liberal critics, meanwhile, say anything short of a pathway to legalization for undocumented immigrants will effectively consign them to the status quo — unwilling to come forward because of the risk of being deported, and unable to fully assimilate into American life.

Some experts also say the nationalization of E-Verify technologies could push undocumented workers further into “shadow economies,” leaving them vulnerable to abuse and unable to pay taxes on their labor.

Frank Sharry, executive director of America’s Voice, an immigration reform organization, said expanding E-Verify would represent an “attrition” approach to removing undocumented immigrants — similar to the call by 2012 GOP nominee Mitt Romney for “self-deportation.” The goal, Sharry said, would be to reduce the size of the undocumented population without offering legal status.

“Trump is fumbling for words to try to broaden his appeal with suburban voters, and the actual policy is incidental at best for him,” Sharry said. He’s “been branded as the candidate of mass deportation, and the terms ‘mass deportation’ and ‘deportation force’ are very unpopular with the very voters he needs to bring home. He’s trying to pretend he’s changing policy when he’s just trying to get rid of the anchor around his neck from the primary that’ll hurt him.”

Philip Rucker contributed to this report.