When President Obama nominated Goodwin Liu to be an appeals court judge earlier this year, some on the left cheered. Mr. Obama had previously picked a succession of nominees who they believed were too centrist to counter the conservatives appointed during the Bush administration, they said, but finally he had selected a liberal legal rock star.

But the effort to confirm Mr. Liu, a law professor at the University of California, Berkeley, whose confirmation hearing is scheduled for Friday, has become the toughest fight over any of Mr. Obama’s appeals court nominees. It could be a harbinger for how a strongly liberal pick to succeed Justice John Paul Stevens would play out.

Asked this week whether Republicans would use a filibuster to block a vote on Mr. Obama’s coming Supreme Court nominee, Senator Jeff Sessions of Alabama, the ranking Republican on the Judiciary Committee, brought up Mr. Liu’s nomination as an example of the kind of candidate who would merit the use of aggressive tactics.

“I promise a fair hearing, and I promise that the nominee will have a chance to explain any criticisms that are raised,” Mr. Sessions said on NBC’s “Meet the Press” on Sunday. “But if a nominee is one that is so activist like Goodwin Liu that’s just been nominated — who’s written that, that the Constitution requires welfare and health care to individuals — if it’s somebody like that, clearly outside the mainstream, then I think every power should be utilized to protect the Constitution. We’ll not confirm somebody like that.”

Supporters of Mr. Liu, nominated for the United States Court of Appeals for the Ninth Circuit, in San Francisco, argue that critics have exaggerated his writings to portray him as an ideologue. A 2008 Stanford Law Review article by Mr. Liu about welfare rights, which Mr. Sessions referred to, focused on small-scale disputes over Congressionally enacted programs — like “invalidating statutory eligibility requirements” — not creating welfare programs based on judicial fiat.

Still, Mr. Liu has been more open in expressing liberal political views — like support for affirmative action and same-sex marriage — than Mr. Obama’s other appeals court nominees. In that sense, he is arguably the first Obama nominee who is the equivalent of some of the most controversial nominees by Mr. Bush, several of whom Democrats delayed or blocked.

Mr. Liu also earned conservative enmity by criticizing Mr. Bush’s two Supreme Court appointees, Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr. And a book he co-wrote argues that judges should interpret the Constitution “in light of the concerns, conditions and evolving norms of our society” — an approach some conservatives say enables judges to impose their own political values.