In a much-anticipated move, President Donald Trump on Wednesday signed a directive ordering federal funds to be diverted to begin building a massive wall along the U.S.-Mexico border.

While Trump long promised during the presidential campaign to build a wall, he insisted Mexico would pay for it, and the decision to use taxpayer funds and later seek reimbursement from Mexico is a hugely contentious move.

During a visit to the Department of Homeland Security, Trump signed two immigration-themed executive orders on Wednesday. One covers starting his promised border wall project, as well as expanding resources for DHS and Immigration and Customs Enforcement. The second seeks to strip federal funding for “sanctuary cities.”

Trump told DHS employees after a town hall meeting that one of his orders directs the “immediate construction of a border wall,” which he said is “so badly needed.”

“We are going to restore law and order,” Trump said, then immediately recognized U.S. Border Patrol and Immigration and Customs Enforcement officers in the room. The line is consistent with his self-designation as the “law-and-order” candidate in the 2016 presidential race.

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He declared Wednesday the day America “gets back its borders,” adding any country that lacks borders cannot be a country.

The executive order ordering its construction it instructs federal agencies and departments to compile lists of all “direct and indirect” federal funds that have gone to the Mexican government over the past five years. The secretary of State is directed within the next 60 days to take that data and “submit to the president a consolidated report reflecting the levels of such aid and assistance that has been provided annually, over each of the past five years.”

The move comes one day before the new president will fly to Philadelphia to address House and Senate Republicans at their annual retreat, and it means they will face votes in the coming months on how to fund Trump’s top campaign pledge.

“Building this barrier is more than just a campaign promise. It’s a common-sense first step to really securing our porous border,” White House Press Secretary Spicer told reporters. “This will stem the flow of drugs, crime, illegal immigration into the United States. And, yes, one way or another, as the president has said, Mexico will pay for it.”

Trump clearly felt pressure to make some kind of progress on the wall, which he has promised will be built using reinforced steel and concrete, along the nearly 2,000-mile border. After all, he stood on stages for over a year at campaign rallies listening to — and egging on — crowds chanting “build the wall!” Often, Trump would ask his supporters, “Who’s going to pay for the wall?” In unison, and at great volume, they would reply with, “Mexico!”

Who’s going to pay?

Since, however, the 45th chief executive has backed off that claim, saying recently that Congress — meaning, U.S. taxpayers — will pay for its construction. The Mexican government, Trump now says, will reimburse the United States at an unspecified date down the road; Mexican officials scoff at this.

House and Senate Republicans, some at first skeptical of the cost of first building the structure then maintaining and patrolling it, now appear on board with the plan — despite Trump’s own estimate it could cost $8 billion and independent projections as high as $25 billion.

“The chairman and the committee are committed to securing our borders and keeping our nation safe,” House Appropriations Committee spokeswoman Jennifer Hing said Wednesday, referring to Chairman Rodney Frelinghuysen, R-N.J. “We will work with the Administration and take a close look look at any funding proposal when it is submitted to Congress.”

No formal funding request has been submitted to Capitol Hill; after all, the administration is only in its fifth full day. Spicer on Monday told reporters the wall is a “serious priority” for Trump, noting his boss “has already started to work with Congress on the appropriations avenue of that.”

Trump is “doing everything he can to direct agencies and Congress to commence with that work as soon as possible,” Spicer added.

As Republicans gather to strategize in Philadelphia, how to fund the wall is sure to be a topic of conversation. There are several ways lawmakers could allocate the money, though Trump has made clear he wants to fast-track the project.

John Thune of South Dakota, the No. 3 Senate Republican, reiterated Wednesday that “money would have to be appropriated” for the project.

‘On the same page’

Though Trump and Speaker Paul D. Ryan have several disagreements over immigration policy, the Wisconsin Republican recently signaled his willingness to go along with the project, saying he and Trump are “on the same page” on border security.

“I’m in favor of securing the border. And I do believe that you have to have physical barriers on the border,” the Wisconsin Republican told Fox News’ Bret Baier on Nov. 10. “I will defer to the experts on the border as to what is the right way to actually secure the border.” He had been asked if he supports Trump’s wall proposal.

Ryan has placed securing the U.S.-Mexico border as among the House Republican’s top agenda items under Trump’s tenure.

The speaker listed securing the border, along with overhauling the tax code and repealing and replacing the 2010 health care law, as top agenda items for the GOP under a Trump administration.

Nita Lowey of New York, the ranking Democrat on the House Appropriations Committee, earlier this month accused Republicans of planning to “force U.S. taxpayers to pay tens of billions of dollars for a border wall.”

Alluding to Trump’s campaign-trail assurances that Mexico would be on the hook, Lowey said using U.S. taxpayer funds would amount to “a broken promise.” She dubbed the border wall “a colossal waste of money, and a boondoggle of historic proportions.”

Democratic leaders pushed back immediately on the wall.

“This wall won’t work,” said House Minority Whip Steny H. Hoyer of Maryland, who joined members of the Congressional Hispanic Caucus in response to the actions. He said while all lawmakers agree there need to be improvements to immigration laws and security of all of the country’s borders, this was not the way to do it.

The Maryland Democrat’s message to the president was: “Don’t take political slogans which have not been thought through, which will not be effective, and turn them into policy.”

Other Democrats were more colorful in their language.

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Mexican President Enrique Peña Nieto will be in Washington next week to discuss immigration, the proposed wall, and other topics with Trump.

Trump’s team says it is surprised that lawmakers, analysts, pundits or journalists were surprised to learn about a Mexican reimbursement plans after months and months of the campaign promises, even though there was hardly any mention of it during the campaign. GOP members have expressed the sentiment.

“As far as having Mexico reimburse, you know, that’s something that the administration will work on,” Rep. Mario Diaz-Balart, R-Fla., an Appropriations Committee member, said 11 days before Trump was sworn in Friday. “But I’ve always assumed we’re going to have to spend money.”

At least one GOP congressman doesn’t think the wall is a good idea. Texas Rep. Will Hurd hails from the state’s 23rd District, which he says includes more than 800 miles of U.S.-Mexico border (more than any other district).

Hurd called the wall the “most expensive and least effective way to secure the border,” in a Wednesday statement, citing “unique geographical, cultural, and technological challenges” that can’t be addressed by a monolithic solution.

Even if Trump secures funding from Congress to start or even complete the wall’s construction, the project faces many hurdles. For instance, many large sections would cut through privately owned lands, raising the likelihood that its construction could be slowed by bureaucratic red tape and even legal challenges.

Other barriers could come from within Trump’s own Cabinet.

Homeland Security Secretary John Kelly told the Senate Homeland Security Committee this month that a physical wall alone is not enough to secure the border. Kelly, a former U.S. Southern Command chief, told the panel that “layered defenses” and human beings are necessary to properly patrol the lengthy border.

Lindsey McPherson, Rema Rahman, Ryan McCrimmon and Gopal Ratnam contributed.