(CNN) Republican Sen. Susan Collins said Tuesday she will vote against Chad Readler, President Donald Trump's nominee for the Sixth Circuit Court of Appeals who has come under fire from Democrats for being a lead lawyer for the Justice Department when it declined to defend key provisions of the Affordable Care Act in response to a lawsuit filed by Republican state attorneys general.

Collins, a rare GOP defender of Obamacare, cited in her statement announcing her decision Readler's role in the case as reason for opposing him.

"As the Acting Assistant Attorney General of the Justice Department's Civil Division, Mr. Readler was both a lead attorney and policy advisor in the Department's decision not to defend the Affordable Care Act, including its provisions protecting individuals with pre-existing conditions," she said. "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."

Readler faces a key procedural vote at 4 p.m. ET on Tuesday when Republicans will try to break a Democratic filibuster of him. It's not clear if any other GOP senators will join her in opposing Readler. When Sen. John Thune of South Dakota, the Republican whip, was asked if Republicans would hold together to advance the nominee, he replied, "hopefully we'll have enough."

He later said he expects to get the simple majority needed.

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