On Monday, President-elect Donald J. Trump will return to the White House with a raft of boundary-pushing executive orders expected to curtail immigration, hasten deportations, and unleash oil and gas drilling, among other efforts.

On the opposite side of the country, an old adversary awaits him: the famously liberal U.S. Court of Appeals for the Ninth Circuit, where progressive lawyers repeatedly persuaded a court dominated by judges nominated by Democratic presidents to stymie much of his agenda during his first term.

But this time, Mr. Trump’s team is more prepared for legal battle. The incoming White House team has had years to plan for the fights that are sure to take place in the circuit’s headquarters, a 120-year-old Beaux-Arts courthouse in downtown San Francisco. And their arguments could receive a warmer reception, now that 10 of the appeals court’s 29 judgeships have been filled with Mr. Trump’s own nominees.

Even before Mr. Trump’s first-term appointments, the court was beginning to shed its identity as a bastion of liberal jurisprudence, according to Jeremy Fogel, a retired federal judge who served in the Northern District of California. “The court has really changed,” he said. “It’s not the old Ninth.”

The Ninth Circuit is the largest of the country’s 12 regional appeals courts in terms of judges, population and square miles. Its sprawling jurisdiction covers nine Western states, including the technology hub of Silicon Valley and more than 500 miles of the southwestern border, areas where Mr. Trump’s policy plans could loom large.

Most of the states within the Ninth Circuit have Democratic governors and attorneys general, making the court a crucial referee for state challenges to policies from any Republican White House. California alone filed more than 100 suits during Mr. Trump’s first term, when Democratic attorneys general from Western states won a number of victories in the circuit that slowed or even stopped some of Mr. Trump’s agenda.

The Ninth Circuit initially sided with Minnesota and Washington to block Mr. Trump’s attempt to stop travel to the United States from seven majority-Muslim countries. The following year, its judges ruled in favor of California to preserve President Barack Obama’s program, known as Deferred Action for Childhood Arrivals, or DACA, that protects some undocumented immigrants who were brought to the country as children.

And the circuit figures largely in the opposition’s emerging second-term legal strategies as well.

“During the next four years, there is going to be a lot of litigation,” promised Skye Perryman, president and chief executive of Democracy Forward, which brought legal challenges against many of Mr. Trump’s policies during his first term. “There’s no question that there are people and communities in the Ninth Circuit who are going to seek relief from policies that this administration will seek to push.”

Sitting above the trial courts and below the Supreme Court, the appellate courts will play a crucial role in deciding when and how much of Mr. Trump’s second-term agenda can be implemented. Appellate courts have the power to uphold or overturn trial-court rulings on the legality of administration policies.

Beyond that, appeals court procedural decisions — such as emergency orders to “stay,” or stop, implementations, nationwide injunctions that can block actions from coast to coast, and “rehearings” that force judicial reconsideration — could mean the difference between the swift and decisive action that Mr. Trump has promised and something closer to what happened in the early months of Mr. Trump’s first term.

Then, for instance, the travel ban was tied up for months in the courts before the Supreme Court finally signed off on a limited version in December 2017. DACA remains an active federal program.

Even with the presence of more Trump nominees on the Ninth Circuit’s bench, the court will almost certainly remain the venue of choice for challenges to administration policies, Mr. Fogel said.

One reason is the sheer number of trial-court judges under the Ninth Circuit’s purview who might be perceived as more skeptical of policies like an end to automatic citizenship for babies born on U.S. soil or the mass deportation of undocumented immigrants. Eighty-eight of those judges were chosen by Democratic presidents; 21 were chosen by Republicans.

But the legal terrain has changed since 2017. Mr. Trump’s advisers have learned from their first-term court battles and four years of challenging Biden administration policies through nonprofits backed and staffed by Mr. Trump’s allies, such as America First Legal Foundation, run by Stephen Miller, an immigration hard-liner and the incoming White House deputy chief of staff for policy.

They have also benefited from Mr. Trump’s increasing dominance within the Republican Party, which has brought in seasoned legal talent who see themselves as accelerants, not guardrails, for his agenda. They are unlikely to commit the blunders of his first term, such as the first version of Mr. Trump’s travel ban. The courts saw dubious legal footing and chaotic implementation in many of Mr. Trump’s initial gambits.

The Trump team also has fresh backup. The Supreme Court, with its 6-3 conservative supermajority and three Trump nominees, will still have the last word on any appealed case it chooses from the Ninth Circuit.

Muneer I. Ahmad, a professor at Yale Law School who has represented DACA recipients, said the Supreme Court could also make it harder for lower-court judges to strike down Mr. Trump’s policies nationwide, by tightening the parameters on injunctions — court orders by a single judge that block the federal government from enforcing its policies.

“Even a favorable district court may not be as powerful this time around,” Mr. Ahmad said.

Mr. Trump is coming into his second term with greater awareness of what it will take for his policies to survive, said Mike Davis, a former Republican counsel to the Senate Judiciary Committee who now heads the Article III Project, an advocacy group created to maintain conservative momentum in the courts.

“Trump understands what he can and can’t do this time,” he said. “When you’re president, you have a lot of power, but you don’t control the courts or Congress. It’s different from being chief executive of a privately owned company.”

The Ninth Circuit has changed as well. Mr. Trump’s judicial appointments tightened the balance on the appeals court from an 18-to-7 split between Democratic and Republican nominees in 2017 to a 16-13 divide today.

The court’s shifting composition is especially important because of a quirk in its internal rules. Like the other 11 regional circuit courts, the Ninth Circuit reviews trial-court decisions using three-judge panels. If the other judges disagree with the panel’s decision, they can vote by majority to review it with a larger “en banc” panel. For other circuits, such reviews involve every active judge on the court. For the large Ninth Circuit, such panels are made up of 10 randomly selected judges, plus the chief judge of the circuit, a Clinton nominee.

So the Trump administration stands a significantly better chance of drawing a Republican-majority panel than it did in 2017, said Erwin Chemerinsky, dean of the Berkeley School of Law at the University of California.

To be sure, in what is certain to be a busy legal season, other appellate courts will also be called on to examine the new president’s policies. New England’s First Circuit, the Fourth Circuit in the South Atlantic region and the D.C. Circuit in the nation’s capital all have majorities of Democratic nominees on their benches. Between 2005 and 2016, any senator could effectively veto the nomination of any circuit-court judge in their home state, a power they still hold for district-court judges. That means that the composition of each court tends to reflect the politics of the region they oversee.

But in drawing Mr. Trump’s rancor, the Ninth seems to stand alone. He once called it a “terrible, costly and dangerous disgrace.” That view appears to be consistent with a longstanding conservative perception of the court as liberal to an extreme.

That perception was once well-taken: An avatar of that earlier era, Judge Harry Pregerson, a nominee of Jimmy Carter, declared during a 1979 Senate confirmation hearing that he would “follow my conscience,” even if it conflicted with the letter of the law. In 1992, Judge Pregerson was among the Ninth Circuit judges who fought a late-night battle with the Supreme Court, issuing four stays in a failed attempt to stop an execution by cyanide gas. Another unapologetic Ninth Circuit liberal, Judge Stephen Reinhardt, offered a distinctly nonlegal defense for the barrage of his rulings that were overturned by the Supreme Court. The nation’s highest court, he said, “can’t catch them all.”

But Judge Pregerson died in 2017, Judge Reinhardt died the year after. The contemporary Ninth Circuit is more moderate than Mr. Trump’s ire would leave his supporters to believe.