The Senate on Wednesday confirmed a controversial federal circuit court pick who backed a lawsuit challenging the Affordable Care Act.

Senators voted 52-47 on Chad Readler's 6th Circuit nomination, with GOP Sen. Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (Maine) siding with Democrats to oppose him.

Readler, who previously worked as an assistant attorney general for the Civil Division at the Department of Justice, ran into controversy over a brief he filed last year supporting a lawsuit filed by Texas and other states seeking to strike down the Affordable Care Act.

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Collins said earlier this week that she would not support Readler, citing his work tied to the court case and his role in the Trump administration's "failure to defend provisions under current law that protect individuals with pre-existing conditions."

"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," she added.

Democrats and their outside group allies have gone on offense over Readler's nomination, arguing that supporting his nomination would be akin to voting against protections for individuals with pre-existing conditions.

"As this chamber is, by now, no doubt aware, Mr. Readler was the chief cook and bottle washer of the Trump administration’s decision not to defend the health care law in court," Schumer said ahead of the vote on Wednesday.

He added that the vote on Readler would "be remembered ... for a long time. A long, long time."

But Democrats are unable to block court picks on their own after they went nuclear in 2013 to nix the 60-vote filibuster for district and circuit court nominations.

Republicans have focused their energy on confirming Trump's judicial picks, which they view as their best shot at using their majority to have a decades-long impact on the direction of the country.

Appeals courts, in particular, are a top priority for Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.) because the circuit courts hear thousands of cases every year — compared to the Supreme Court, which heard 69 cases during their last term — and often have the final say for states within their jurisdiction.

McConnell praised Readler from the floor this week, calling him "another of President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's impressive judicial nominees."

Democrats would have needed to flip four Republicans if they were going to block Readler's nomination.

In addition to his work on the Affordable Care Act lawsuit, Readler's nomination is considered controversial because Republicans advanced his nomination even though Sen. Sherrod Brown Sherrod Campbell BrownSenate Democrats release report alleging Trump admin undermined fair housing policies Bipartisan praise pours in after Ginsburg's death Emboldened Democrats haggle over 2021 agenda MORE (D-Ohio) didn't return his blue slip.

The blue-slip rule — a precedent upheld by Senate tradition — has historically allowed a home-state senator to stop a lower-court nominee by refusing to return the blue slip to the Judiciary Committee.

How strictly the precedent is upheld is decided by the committee chairman, and enforcement has varied depending on who wields the gavel.

But the use of the blue slip has emerged as a flashpoint during the Trump administration, as several Democratic senators have refused to return their paperwork on circuit court nominees from their home states, setting up a round of fights between Democrats and the White House.

Brown announced last year that he would not return his blue slip. A spokeswoman confirmed Wednesday that he had not returned his blue slip for Readler or Eric Murphy's circuit court nomination. The Senate is also poised to confirm Murphy this week.

“I cannot support nominees who have actively worked to strip Ohioans of their rights. Special interests already have armies of lobbyists and lawyers on their side, they don’t need judges in their pockets," Brown said in a statement earlier this year.