Amul Thapar broke new ground when he became the first federal judge of South Asian descent in 2007, and over the next decade he built a record as a well-respected jurist.

When President Trump nominated Judge Thapar to a seat on a federal appeals court, the American Bar Association unanimously graded him “well qualified.” Yet when the son of Indian immigrants came up for a vote on the Senate floor, not a single Democrat backed him.

The process played out the same way for Neomi Rao, a daughter of Indian immigrants whom the ABA rated “well qualified” but who was confirmed only on a party-line vote, and for Michael Park, a Korean American, and Kenneth Lee, a Korean immigrant.

None of them earned a single Democratic vote.

It’s tough to square that record with Democratic senators’ complaints about a lack of racial diversity or female presence on the federal bench, conservative court watchers say.

“These are some real standouts,” said Carrie Severino, chief counsel at the Judicial Crisis Network. Democrats “are looking at the wrong thing — ‘We are trying to come up with quotas and ratios’ — let’s look at having great judges, men and women, across the spectrum.”

Democrats have reasons for their opposition in each case.

Judge Rao, who replaced Justice Brett M. Kavanaugh on the U.S. Circuit Court of Appeals for the District of Columbia, was dinged for past writings on date rape. Democrats said Judge Thapar was reversed too often by higher courts on criminal and campaign-finance cases.

Judge Park drew complaints for the clients he had as a lawyer, who backed the inclusion of a citizenship question on the 2020 census and challenged college affirmative action programs.

Marge Baker, executive vice president of the liberal People for the American Way, said it comes down to the nominees’ records and whether they can be fair-minded jurists.

“It also comes down to ideology,” Ms. Baker said.

One common theme in Democrats’ opposition is that the nominees were part of the Federalist Society, an organization of conservative- and libertarian-leaning lawyers and scholars.

That was a big factor for Sen. Mazie K. Hirono, a Hawaii Democrat who, while demanding diversity on the bench, has found little to like in the picks Mr. Trump has offered.

“Many of them, regardless of the sex, age or background, have written all kinds of articles or made their positions very clear on choice, LGBTQ rights,” she said. “They share some common traits, large common factors: membership in the Federalist Society, which is a very conservative right-wing organization.”

Ms. Hirono, a member of the Judiciary Committee, has supported 54% of Mr. Trump’s female and minority judicial nominees.

Sen. Cory A. Booker of New Jersey, another committee member and a backer of judicial diversity, has supported even fewer of Mr. Trump’s female and minority nominees. The same goes for Senate Minority Leader Charles E. Schumer, New York Democrat and a former member of the committee.

Neither man’s office returned a request for comment.

Mr. Trump does lag behind his predecessors in diversifying the bench.

In his first 2½ years in office, the president won confirmation of nine Asian American judges, five Hispanic judges and three black judges out of a total of 123 — or 14%. Of his nominees, 22% are women.

President Obama, meanwhile, confirmed 38 women to the federal bench during the same period of his term. As far as race, he appointed 19 blacks, 10 Hispanics and seven Asians.

Carl Tobias, a law professor at the University of Richmond, said President Carter in the 1970s led the way in diversifying the bench and that President Clinton and Mr. Obama expanded it in terms of race and sexual orientation.

Mr. Trump has not had any LGBTQ judicial nominees confirmed.

Curt Levey, president of the Committee for Justice, said minorities tend to be Democrats, so it is no surprise that a Republican president would have lower diversity numbers.

“I don’t think you can draw a lot of conclusions from comparing Obama’s numbers to Trump’s numbers,” he said.

Although Democratic senators have called for more minorities, he said, they don’t always vote to confirm them.

“It’s rhetoric, frankly,” Mr. Levey added.

Mike Davis, president of The Article III Project, which backs the president’s nominees, said Democrats have a history of resisting minority nominees from Republican White Houses.

Perhaps the most prominent was Justice Clarence Thomas, nominated by President George H.W. Bush. But another major battle erupted after his son, President George W. Bush, nominated Miguel Estrada for a seat on the U.S. Court of Appeals for the District of Columbia.

Originally from Honduras, Mr. Estrada immigrated to the U.S. and graduated from Harvard Law School.

Democrats feared Mr. Estrada, with judicial experience under his belt, would eventually become the first Hispanic Supreme Court justice. Staff for Sen. Richard J. Durbin, a member of the Judiciary Committee, called Mr. Estrada “especially dangerous” because “he is Latino.”

Fueled by those fears, Democrats launched the first-ever partisan filibuster to defeat a judicial nominee. The practice ended only after use of the “nuclear option” rules change in 2013.

Mr. Davis said Democrats’ opposition to Mr. Trump’s minority nominees is political.

“Democrats know they cannot afford to let the Republicans take any votes out of their coalition,” he said.