Sen. John Cornyn, R-Texas, criticized parts of Koh's record before Thursday's vote. As he did during a July confirmation hearing, Cornyn zeroed in on a ruling Koh made last year against the federal government, requiring law enforcement agencies to get a warrant before they obtain location data generated by a subject's cellphone.
"This is an example of the kind of judicial activism that portrays a lack of regard for the plain text of the Constitution and the laws passed by Congress ... in favor of what the judge views as better policy," Cornyn said.
Grassley didn't speak about Koh on Thursday, but in a statement filed with the committee acknowledged fellow Republicans' worries about Koh's ruling on the electronic data and privacy case.
"I share some of those concerns," Grassley wrote.
"I'm willing to vote for Judge Koh today so we can move her nomination out of this committee," he said. "But I want to look further into this matter and be clear I'm not making any commitments about a floor vote I'd cast on this nomination."
Other Democrats, several legal experts and advocates for adding diversity to the federal bench have hailed her nomination and pushed for her confirmation.
Koh is the first Korean-American to serve as a U.S. District Court judge. She would be the second to serve on a federal appeals court and the first Korean-American woman.
With Republicans holding a 54-46 majority in the Senate, Koh will need GOP support to win confirmation.
Carl Tobias, a law professor who specializes in federal courts and judicial selection at the University of Richmond's School of Law, said in an email he's "cautiously optimistic that the Senate will confirm her."
He noted that Koh, a former Santa Clara County Superior Court judge who was confirmed to the District Court by a 90-0 vote in 2010, has enjoyed GOP support in the past.
"She is a highly qualified, mainstream nominee who enjoys strong support from many Republicans, such as Gov. (Arnold) Schwarzenegger, who appointed her to the Superior Court, and Stanford Law Professor Mike McConnell, formerly a 10th Circuit judge," Tobias said.
The Senate has not scheduled a confirmation vote yet, and Tobias said it's not clear when that will happen.