WASHINGTON (AP) — The American Civil Liberties Union filed legal challenges against former President Donald Trump 's administration more than 400 times during his time in the White House, helping to halt an array of policies, including separating immigrant children from their parents.

The ACLU isn't conceding that Trump will beat President Joe Biden this year. But it's publishing a blueprint on how it plans to respond to a second Trump term given his promises to go much further on immigration, with calls for mass raids and the largest deportation operation in U.S. history.

Advocacy groups are making contingency plans to try and tie up Trump's priorities in court or through the workings of government. Trump's allies, mindful of the resistance he faced in the White House and anticipating the chance to remake huge swaths of government, have prepared policy books and staffing plans of their own, including one effort known as "Project 2025."

The ACLU shared a memo offering possible responses on immigration policy with The Associated Press ahead of its formal release on Thursday.

"This is really kind of the sequel on the earlier work that we did fighting off the worst of the Trump abuses," said Anthony Romero, the group's president.

Here's a look at the ACLU's strategy and how it might play out.

What's Trump planning?Immigration is a centerpiece of the former president's campaign to reclaim the White House.

Trump has endorsed major arrest operations against people in the country illegally with the help of the National Guard. He's talked of opening sprawling detention camps and fast-tracking deportations.

He's also discussed ending automatic citizenship for anyone born in this country, a guarantee in the 14th Amendment that some conservatives argue shouldn't apply to the children of people in the U.S. illegally. Trump may additionally revive some of his first-term policies, like banning entry into the U.S. of people from some majority-Muslim countries or separating immigrant families anew.

Karoline Leavitt, a campaign spokeswoman for the former president, said Trump will "act to secure the southern border and reimplement his prior effective policies to protect our homeland, no matter what challenges are thrown his way or no matter how long it takes."

How will the ACLU respond?With lawsuits. Likely lots of them.

Trump has suggested he can streamline arrests and deportations by evoking the Alien Enemies Act of 1789, which could allow him to unilaterally detain and deport some noncitizens. The ACLU counters that the act only gives the president limited use of such powers during a "declared war," or an "invasion or predatory incursion" involving a foreign nation or government.

It further argues that carrying out Trump's plans will violate constitutional protections against unreasonable search and seizure, including arrests and detentions without a specific reason to detain a certain individual.

Separately, Trump has pointed to the Insurrection Act, which gives the president powers to use the military as a domestic police force, and suggested that troops could help handle his immigration plans' complicated logistics. But the ACLU says the Posse Comitatus Act, which dates back to 1878 and which Congress has moved to strengthen more recently, forbids using the military in civilian law enforcement.

The memo says Trump's pledges to end birthright citizenship, meanwhile, contradict constitutional guarantees of citizenship to people born in the United States without regard for parentage and that the Supreme Court has affirmed that those guarantees applied to U.S-born children — even if their parents didn't have citizenship rights.

Regarding the potential separation of immigrant families, the ACLU settled with the federal government last year a case it initiated against the Trump administration in 2018, opposing the separating of a Congolese woman being held in a detention facility in California, from her then-7-year-old daughter, who was in a Chicago facility. Any attempt by a new Trump administration to restart the policies would contradict the court-ordered settlement agreement, the ACLU argues, and give it the grounds for new legal challenges.

How could Trump respond?The conservative Heritage Foundation has helped create a more than 1,000-page "Project 2025" handbook. It includes scores of proposed actions on immigration and could potentially make a new Trump White House more prepared to overcome lawsuits on the issue than the first one was.

"The second Trump administration, if there is one, will be better prepared," said Stephen Yale-Loehr, a professor of immigration law practice at Cornell University.

He noted that the first Trump administration often saw its policies halted by rulemaking and procedural mistakes that it could fix this time around — it could use past legal decisions to find workarounds.

"Both sides have seen the litigation battles, and seen how the courts have ruled," Yale-Loehr said.

Did lawsuits work during Trump's first term?Yes, to a point.

Legal challenges helped stop the Trump administration from separating immigrant families at the border and degrading immigration protections offered under Temporary Protected Status and the Deferred Action for Childhood Arrivals program, whose recipients are commonly called "Dreamers."

The group notes that when its challenges weren't ultimately successful — like when the Supreme Court reversed injunctions against the Trump administration's ban on travelers from several majority Muslim countries — they nonetheless forced officials to scale back their intentions.

Lucas Guttentag, a Stanford University law professor who founded the ACLU's Immigrants' Rights Project, said that though the Supreme Court now has a 6-3 conservative majority, it and lower appeals courts may remain skeptical of the constitutionality of some of Trump's top plans.

But he also said that isn't a guarantee others won't be allowed to stand.

"The only foolproof mechanism is to defeat him at the ballot box," Guttentag said.

What about the ACLU's plans beyond lawsuits?The group will urge state and local leaders to help protect against mass deportations by funding legal counsel for immigrants. It also wants them to better cooperate to track large-scale arrests and document racial profiling.

It plans to urge Democratic-led legislatures and city councils to restrict federal government access to their resources for mass detention and deportation efforts.

Romero said the ACLU is identifying "real, clear guardrails, real barriers — at the very least, they're speed bumps — for the Trump administration to get over."

"Litigation takes time," he said, "so if you can preserve the status quo for the longest period of time that is success in our book."

What about Trump's plans beyond immigration?The ACLU will release seven subsequent policy memos responding to Trump's campaign promises on top issues. That includes plans to curb potential abuses of executive power and better safeguard things like LGBTQ rights, reproductive freedom, voting rights and diversity, equality and inclusion protections.

It is set to release each plan weekly leading up to the Republican National Convention, which opens July 15 in Milwaukee.

What about Biden?President Joe Biden on Tuesday announced plans to significantly restrict the number of immigrants seeking asylum at the U.S.-Mexico border. Romero said the ACLU is preparing likely legal challenges against that order. His group repeatedly sued the Biden and Obama administrations over immigration policy in the past — though not at the pace of its challenges to Trump's White House.

The ACLU is also planning to release six upcoming issue memos for Biden's reelection bid ahead of August's Democratic National Convention in Chicago.

"There's a stark contrast between Biden and Trump," Romero said, "but there's still an unfinished agenda with Team Biden."