WASHINGTON (AP) - President Donald Trump could have a tough time making good on his threat to deport millions of people living in the U.S. illegally. But maybe that wasn't his point.

Trump's late-night messages promised that starting next week his administration "will begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States. They will be removed as fast as they can."

That was a pronouncement likely to excite his political base just as he was formally announcing his reelection bid Tuesday night. It also scared immigrants in the U.S. illegally - and could deter others from coming.

But it came at a cost.

Trump blatantly exposed an upcoming enforcement operation, potentially jeopardizing the kind of sensitive effort that takes months to plan and relies on secrecy. The president's tweets put new, fresh demands on Immigration and Customs Enforcement, the agency in charge of removals, which is already overwhelmed, lacking staff, funding and detention space for its current work. And any massive roundup that includes deportation of mothers, fathers and children would be sure to spark outrage.

The tweets suggested the start of Trump's re-election campaign is likely to have much in common with his 2016 announcement, when he accused Mexico of sending rapists to the United States and pledged to build a wall and make Mexico pay for it. The rhetoric was widely denounced, yet the tough anti-immigration message struck a nerve with many Americans and ultimately helped carry Trump to victory.

But his tough talk hasn't led to a drop in border crossings since he took office. Trump has threatened a series of increasingly drastic actions as he has tried to stem the flow of Central American migrants, which has risen dramatically despite his hardline policies. He recently dropped a threat to slap tariffs on Mexico after the country agreed to step up immigration enforcement efforts.

The "millions" referred to in Trump's tweets referred to the more than 1 million people in the United States with final deportation orders, according to an administration official who spoke on condition of anonymity to explain the president's tweets.

Pew Research Center recently estimated there are about 10.5 million people in the U.S. illegally, with long term residents outnumbering recent arrivals.

Some in Trump's administration believe that decisive shows of force - like mass arrests - serve as deterrents, sending a message to those considering making the journey to the U.S. that it's not worth coming.

The new acting director of ICE, Mark Morgan, recently signaled a willingness to deport families during enforcement sweeps, though past Trump immigration officials hesitated over concerns about logistics and the public reaction.

U.S. officials with knowledge of the preparations say the operation wasn't imminent; it was to begin in the coming weeks and be nationwide. But ICE officials were not aware the president would make public sensitive law enforcement plans, and it's unclear whether the operation now will go off as planned.

There are routine nationwide enforcement sweeps, usually about two per year, requiring months of planning and are time consuming to pull off. Officers have addresses that are often wrong and don't have search warrants. Immigrants are not required to open their doors, and increasingly they don't. Officers generally capture about 30% of targets.

Plus, ICE needs travel paperwork from a home country to deport someone, so immigrants often end up detained at least temporarily waiting for a deportation flight. The adult population of detainees was 53,141 as of June 8, though the agency is only budgeted for 45,000. There were 1,662 in family detention, also at capacity, and one of the family detention centers is currently housing single adults.

Also, publicizing law enforcement operations can jeopardize officer safety and tip off potential deportees.

When Oakland, California, Mayor Libby Schaaf learned of an operation in Northern California and warned the immigrant community, Trump railed against the disclosure. He suggested then-Attorney General Jeff Sessions look into prosecuting her for obstruction of justice. And the head of ICE at the time, Thomas Homan, said his agency could have arrested more people had she not warned them, calling it an "irresponsible decision."

Immigrant advocacy groups across the country are already getting terrified phone calls from people worried about raids.

"People are always on edge," said Cesar Espinosa, executive director of the Houston advocacy group FIEL. "This obviously reinforces that fear and in a lot of cases it paralyzes people when they can't continue to live their daily lives."

Espinosa and other immigrant advocates encourage families to have an emergency preparedness plan, including a contact to care for children if their parents are arrested and savings to pay for a bond or utility bills. He said he's encouraging all families with a member who is in the U.S. illegally to be prepared.

The American Civil Liberties Union was publicizing its "know your rights" campaign again Tuesday after Trump's tweet.

The record year for deportations was in 2012, under the Obama administration, when 419,384 people were deported. Between 2009 and 2012, the Obama administration deported 1.6 million immigrants. About two million immigrants were deported during the eight years of the George W. Bush administration.

An effort to deport more than a million people rapidly is "a fantasy" said John Sandweg, a former ICE head under Obama.

"ICE is always working at 100 percent of its capacity. The president wants to create this illusion that he's let go of the reins that other administrations were holding but that's just not true."

Meanwhile, hundreds of thousands of people, mostly Central American families, are crossing the border monthly, highs not seen for more than a decade. There are holdups in every agency that manages immigration, including a backlog of more than 700,000 immigration cases. Acting Homeland Security Kevin McAleenan has asked for $4.5 billion in supplemental aid to help agencies cope.

The hot summer weather usually deters some people from coming. A senior Mexican official said Monday that, three weeks ago, about 4,200 migrants were arriving at the U.S. border daily. Now that number has dropped to about 2,600.

ICE did not comment on Trump's tweets, but said in a statement it "will continue to conduct interior enforcement without exemption for those who are in violation of federal immigration law."

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