Courts restrained Trump in first term. Will they 'check' his power again?

When Donald Trump assumes the presidency for a second time, top Democrats say he will have "no guardrails" and Supreme Court Justice Sonia Sotomayor has warned he will be a "king above the law."

"Ironic isn't it? The man in charge of enforcing laws can now just break them," Sotomayor claimed in her dramatic dissent in the case Trump v. U.S. in July.

Republican control of Congress would give Trump significant leeway to advance his priorities and undertake a sweeping overhaul of American government and society.

A six-justice conservative majority on the Supreme Court will potentially provide legal cover, and its decision in June to enshrine presumptive immunity for official presidential acts could well embolden Trump in ways he hadn't been before.

But Trump's agenda and his personal impulses won't proceed entirely unchecked.

Trump's appointments to lead executive agencies will require Senate confirmation and are not guaranteed. During his first term, several picks had to withdraw from consideration after stiff opposition from within his own party.

Presidential executive orders and Trump agency regulations will have to comply with the Administrative Procedures Act, an arcane 1946 law requiring a "notice and comment period" for policy changes and proof that public input was considered.

"We will use all the tools at our disposal -- including aggressive litigation -- to ensure that key consumer and other regulatory protections remain intact," said Robert Weissman and Lisa Gilbert, co-presidents of Public Citizen, a progressive advocacy group that has sued Trump in the past.

During Trump's first term, more than 246 agency regulations, guidance documents and memoranda were challenged in federal courts. The administration won 54 of the cases, a 22% success rate, according to data from the Institute for Policy Integrity at the NYU School of Law.

In 192 cases, a court either ruled against the Trump agency or the agency withdrew the controversial action after being sued. Many of those losses involved a violation of the APA.

The Supreme Court notably struck down Trump's decision to end the Deferred Action for Childhood Arrivals (DACA) program; blocked the White House effort to include a citizenship question on the 2020 census; and invalided the initial Trump travel ban targeting predominantly Muslim countries.

"I don't think people paid enough attention to the fact that Trump heeded the Court and agreed to rewrite the 'Muslim ban' three times so that it would pass constitutional muster," said Sarah Isgur, a former Trump Justice Department official and an ABC News legal contributor.

The American Civil Liberties Union and other civil rights groups have vowed to aggressively bring fresh litigation against Trump's agenda.

"When President Trump targets immigrants, dissidents, and his political opponents -- we will challenge him in the courts, at the state legislatures, and in the streets," Anthony D. Romero, executive director of the ACLU, said in a statement.

The group filed 434 actions against the first Trump administration, winning many cases even before Trump-appointed judges.

Democrat-led states are also preparing to join the legal fight.

"Let there be no mistake, we intend to stand with states across our nation to defend our Constitution and uphold the rule of law," said Gov. Gavin Newsom of California, which filed more than 100 lawsuits against the first Trump administration.

(By way of comparison, Texas sued the Obama administration 48 times during his two full terms, according to the Texas Tribune.)

Illinois Democratic Gov. JB Pritzker this week warned, "To anyone who intends to come take away the freedom and opportunity and dignity of Illinoisans, I would remind you that a happy warrior is still a warrior. You come for my people, you come through me."

The expected onslaught of litigation will collide with a federal judicial system packed full of judges that Trump appointed -- more than 200 nominees in four years surpassing the total elevated by then-President Barack Obama in eight years.

Trump appointed three justices to the Supreme Court bench -- justices Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett -- and may get a historic opportunity to nominate even more should a current justice retire or die. Such a development would consolidate conservative power on the Supreme Court for generations.

"Will the Supreme Court act as a meaningful check on a second Trump presidency? During the first Trump administration they were a real check," said University of Pennsylvania Law Professor Kate Shaw, an ABC News legal contributor. "That, however, was a different court. Trump has now transformed the Supreme Court, and this court is not one that is likely to act as a comparable check."

As for checks on Trump's individual conduct, Chief Justice John Roberts made clear in his landmark opinion in June that no current or former president has absolute immunity from criminal or civil prosecution.

While Roberts said core constitutional powers are fully protected, he said there is only "presumptive immunity" for other "official acts," but left to future potential cases to test the boundaries.

Private conduct has no civil or criminal immunity at all, the court has said. In 2020, Trump was forced to comply with a grand jury subpoena for his financial records. Two years later, the court said Trump had to turn over his taxes to a House committee.