Obama's 2010 nomination of Liu to California's Ninth Circuit Court of Appeals foundered in the face of a Republican filibuster amid speculation he was destined to be the first Asian-American appointed to the top court.
Liu's potential as a Supreme Court nominee was "mentioned" in meetings among legal activists who opposed his nomination, said Curt Levey, executive director of the Washington-based Committee for Justice, which usually aligns with Republicans on judicial nominations.
Republicans twice blocked Senate votes on Halligan's confirmation to the District of Columbia Circuit before Obama withdrew her nomination on March 22. Senate Republicans criticized her work while New York's solicitor general on a lawsuit by the state against the gun-manufacturing industry.
Halligan, who was nominated in September 2010 for the appeals court, "is certainly someone who could easily be on President Obama's short list for a Supreme Court nomination," said Douglas Kendall, president of the Washington-based Constitutional Accountability Center, which usually lines up with Democrats on judicial nominations.
Blocking nominees at the appeals level doesn't necessarily mean they will be kept off the Supreme Court.
Clinton's nomination of Elena Kagan in 1999 to the D.C. Circuit Court of Appeals never received a vote in the Senate. That didn't prevent Obama from nominating her to the Supreme Court in 2010 after she served in his administration as solicitor general.
She was the first justice to go on the court without experience as a sitting judge since then-Assistant Attorney General William Rehnquist and lawyer Lewis Powell were confirmed in 1971.
Gerhardt believes that wasn't a coincidence. Sending a message that pre-emptive strikes won't necessarily thwart a nomination to the Supreme Court was "an added incentive" for the choice of Kagan, he said.