Romney, who faced off against Obama in the 2012 presidential race, cast the lone GOP no vote against Judge Michael Truncale, who was ultimately confirmed to the Eastern District of Texas by a 49-46 margin.
Truncale raised eyebrows in 2011 when he called Obama an "un-American imposter."
He later told the Senate Judiciary Committee that he was "merely expressing frustration by what I perceived as a lack of overt patriotism on behalf of President Obama," adding that he did not subscribe to "birtherism."
However, Romney found enough fault with the original remark to vote against Truncale’s nomination.
"He made particularly disparaging comments about President Obama. And as the Republican nominee for president, I just couldn't subscribe to that in a federal judge," Romney told Politico in a brief interview.
"This was not a matter of qualifications or politics. This was something specifically to that issue as a former nominee of our party," he added.
Romney has emerged as one of the few GOP critics of Trump, often hitting the president over his rhetoric. He most recently bucked the White House by opposing its plans to nominate Herman Cain to the Federal Reserve Board.
Though he votes in line with the president more often than not, he is more likely to oppose the White House than most other Republican senators, according to a tally compiled by FiveThirtyEight.