California's top state appellate lawyer is poised to make his U.S. Supreme Court debut in just a few weeks in support of shielding hundreds of thousands of undocumented immigrants from deportation under the Obama-era program known as Deferred Action for Childhood Arrivals.

Solicitor General Michael Mongan is requesting to split 40 minutes with Gibson, Dunn & Crutcher's Theodore Olson, who would argue for certain individual DACA recipients. Mongan would represent the interests of 20 states—including California, New York, Connecticut, Delaware and Pennsylvania—that are arguing against the Trump administration's efforts to rescind the program.

Mongan would make his rookie appearance before the high court in one of the new term's most visible cases, one that crystalizes the ongoing fight between the White House and Democrats over immigration and citizenship. But he is no stranger to the nation's biggest legal stages.

Named California's solicitor general just a little over two months ago, Mongan argued for the state in defense of DACA before the U.S. Court of Appeals for the Ninth Circuit in May 2018. Six months later, the Ninth Circuit upheld a lower court's injunction blocking the Trump administration's attempted rollback of DACA, which temporarily exempts certain undocumented immigrants brought to the U.S. as children from deportation.

The Supreme Court is expected soon to outline its argument plans for the DACA case. It's not uncommon in consolidated cases to let a variety of lawyers argue in support of various positions.

Kyle Hawkins, the Texas state solicitor, has asked the Supreme Court for 10 minutes of argument time to advance the interests of Republican-led states who are supporting the Trump administration's move to end DACA. Noel Francisco, the Justice Department's solicitor general, is expected to argue for the U.S. government.

Mongan, while serving as a deputy solicitor general, was lead counsel for the state and some of its largest cities on an amicus curiae brief challenging the federal government's ultimately unsuccessful attempt to place a citizenship question on the 2020 U.S. Census.

A 2006 graduate of Stanford Law School, Mongan clerked for Judge Merrick Garland of the U.S. Court of Appeals for the D.C. Circuit and, later, U.S. Supreme Court Justice David Souter. He served as deputy counsel to Vice President Joe Biden before joining Munger, Tolles & Olson in 2010.

Mongan went to work for California Solicitor General Edward DuMont in 2014 and served as a deputy and supervising deputy until taking the reins of the 10-attorney office this summer after DuMont resigned. DuMont, a former Wilmer Cutler Pickering Hale and Dorr partner, has not said what his plans are.

In the DACA case, Mongan is attorney of record for California, which filed its merits brief on Sept. 27 in the Supreme Court. The team working with Mongan includes Michael Newman, senior assistant attorney general, and deputy solicitors general Samuel Siegel and Joshua Patashnik. The solicitors general for Maine, Maryland and Minnesota were on the states' brief with the California team.

Ted Olson Gibson Dunn's Ted Olson. Credit: Diego M. Radzinschi / ALM

The Gibson Dunn team includes Theodore Boutrous Jr., global co-chair of the firm's litigation group, and Stuart Delery, a Washington-based partner who formerly was a leading Obama-era Justice Department lawyer.

"Executive power is important, and we respect it," Olson, a former George W. Bush administration U.S. solicitor general, told The New York Times last week. "But it has to be done the right way. It has to be done in an orderly fashion so that citizens can understand what is being done and people whose lives have depended on a governmental policy aren't swept away arbitrarily and capriciously. And that's what's happened here."

When Rookies Take Bat at the US Supreme Court

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Mike Scarcella contributed reporting from Washington.