WASHINGTON, D.C. — President Donald Trump’s bold but vague pledge to deport “millions” of undocumented immigrants starting next week came on the eve of his reelection kickoff rally Tuesday night in Florida — and it vastly overstates the number of likely deportees and the ability of federal agents to round them up.

Like many of Trump’s pronouncements, his tweet may be more about political symbolism and stirring up public attention and anger than setting policy or issuing clear orders to federal authorities. It’s not yet clear if a plan actually exists for mass arrests and removals on the scale and speed that Trump suggests.

Trump tweeted Monday night that Immigration and Customs Enforcement agents “will begin deporting the millions of illegal aliens who have illicitly found their way into the United States … as fast as they come in,” and he called on congressional Democrats to address the “border crisis.”

An administration official said Tuesday that more than 1 million migrants face deportation orders and “remain at large.” Many have long hidden from federal agents, or have sought refuge in churches and other sanctuaries, and it’s unclear how many ICE agents could find.

The removal orders “were secured at great time and expense, and yet illegal aliens not only refuse to appear in court, they often obtain fraudulent identities, collect federal welfare and illegally work in the United States,” said the official, who spoke on condition of anonymity to discuss the president’s tweet.

Government data partly contradicts that assertion. A Border Patrol official said in April that less than 2% of all undocumented immigrants detained at the border as part of family units in the previous year were found to have made false claims.

If ICE conducts mass roundups and arrests, the effort may give Trump political bragging rights on an issue critical to the voters who put him in office. But ICE is unlikely to quickly locate and remove vast numbers of migrants.

ICE already has been stretched increasingly thin by the near-record influx of migrants at the southern border, chiefly from Central America, over the past year. Its record of deportations was relatively low even before that, however. In fiscal 2017, which includes Trump’s first six months in office, ICE deported only 226,119 immigrants, according to federal data.

In a statement Tuesday, the White House offered its rationale for going after migrants who are facing deportation orders from immigration courts, which are run by the Department of Justice and are not part of the federal judiciary.

The statement did not say when or how the president’s directive would be carried out.

“Countless illegal aliens not only violate our borders but then break the law all over again by skipping their court hearings and absconding from federal proceedings,” the White House said. “These runaway aliens lodge phony asylum claims only to be no-shows at court and are ordered removed in absentia.”

ICE Acting Director Mark Morgan, who was appointed in May, has indicated that the administration has been looking for ways to arrest and deport immigrants who have been issued final deportation orders, including families.

“It’s real and for the optics,” said a Trump campaign official, who spoke on condition of anonymity. “There are over 1 million here with final deportation orders, and we hardly even pursue them. It’d be good policy and politics to get real about enforcement and getting them out of here.”

As he heads into the 2020 race, Trump appears intent on maintaining his support with a base of voters galvanized by his hard-line approach to immigration even if he has failed to deliver on many of his key campaign pledges.

Trump has yet to build new miles of wall along the southern border, and Mexico has flatly refused to pay for it despite Trump’s improbable promises in the 2016 campaign. He has failed to change federal immigration laws, and rather than shrinking the flow of migrants to the southern border, he has seen it surge dramatically on his watch.

Still, Trump has pushed the issue relentlessly —deploying thousands of U.S. combat troops on the southern border, shutting down large parts of the government for 35 days in a failed attempt to get Congress to provide more money, threatening Mexico with 25% trade tariffs before backing down, and using executive authority to bypass Congress and raid the Pentagon budget for increased border security.

Unlike in 2016, he is now the president and capable of determining federal policy. But even after two and a half years in office, there is often no reliable formula for determining which of Trump’s tweets are likely to materialize in reality.

Kirstjen Nielsen resigned in April as secretary of Homeland Security after balking at a White House proposal to rev up deportations of immigrant families, which senior advisor Stephen Miller and others have suggested would reduce incentives for immigrants seeking asylum in the U.S.

Traditionally, ICE has prioritized criminals for deportation.

And immigrant families detained at the border are often released into the U.S. interior because of court-ordered limits on how long minors can be held in government custody. The Trump administration has sought to change that, detaining thousands of immigrant parents and their children before courts stepped in and ordered the families be reunified.