Sen. Susan Collins (R-Maine) said she will vote against Chad Readler, President Donald Trump’s judicial nominee for the 6th Circuit Court of Appeals, because of his role in the Justice Department’s failure to defend key Affordable Care Act provisions against a Republican-backed lawsuit.

Readler, who was acting assistant attorney general for the Justice Department Civil Division from December 2017 to September 2018, filed a brief last year supporting a lawsuit filed by Texas and other Republican-led states seeking to strike down the Affordable Care Act.

In a rare Justice Department refusal to defend a federal law, Readler called Obamacare’s guaranteed protections for people with pre-existing conditions “unconstitutional.” His brief prompted three Justice Department attorneys to withdraw from the case, and one resigned from the department altogether.

“Rather than defend the law and its protections for individuals with pre-existing conditions ― such as asthma, arthritis, cancer, diabetes, and heart disease ― Mr. Readler’s brief in Texas v. United States argued that they should be invalidated,” Collins said in a statement Tuesday.

“Given Mr. Readler’s role in the government’s failure to defend provisions under current law that protect individuals with pre-existing conditions, I will oppose his confirmation to the bench,” she continued.

Collins, who likely faces a challenging re-election campaign in 2020, is one of several Republican senators who have broken ranks with party leaders on key legislation. She was one of three Republican senators in July 2017 to vote against their party’s health care bill that would have repealed the Affordable Care Act, and said she will vote against Trump’s national emergency declaration for his promised border wall.

But she voted in December 2017 to pass the Republican tax plan, which repeals the Affordable Care Act’s individual mandate, leaving 13 million more Americans uninsured and increasing premiums by 10 percent. She also voted to confirm Supreme Court Justice Brett Kavanaugh.

It’s unclear whether any other Republicans will join Collins against advancing Readler’s nomination, which requires a simple majority in the GOP-held Senate. Sen. John Thune of South Dakota, the Republican whip, told CNN he expects Readler will get enough votes.

Another moderate Republican, Alaska’s Sen. Lisa Murkowski, said she also has concerns about Readler’s involvement in the Texas case.

“I’m trying to find out is to what extent was he involved in some of these matters that are more of concern to me,” she said, pointing to his oversight role.

All Democrats are likely to oppose the confirmation of Readler, who has also supported efforts to weaken voting rights and defended Trump’s ban on transgender people serving in the military.

UPDATE: March 6 ― The Senate confirmed Readler by a vote of 52-47. Collins was the only Republican to vote no.