Senate Majority Leader Mitch McConnell, a Republican from Kentucky, left, speaks as U.S. House Minority Leader Kevin McCarthy, a Republican from California, right, and U.S. President Donald Trump listen before the H.R. 748, Coronavirus Aid, Relief, and Economic Security (CARES) Act, is signed in the Oval Office of the White House in Washington, D.C., U.S., on Friday, March 27, 2020.

Conservative dark money groups are preparing for one of their biggest battles of the year over President Donald Trump's new pick to fill the a seat on a court that once included Supreme Court Justice Brett Kavanaugh.

Trump on Friday announced that he will nominate Justin Walker to the D.C. Circuit Court of Appeals. Kavanaugh was confirmed to the Supreme Court in October 2018 after a bitter fight in the U.S. Senate that included accusations of sexual assault which Kavanaugh vehemently denied.

The fight over Kavanaugh's Supreme Court confirmation included millions of dollars of outside money spent either for or against the controversial judge.

Walker was confirmed in 2019 to the U.S. District Court for the Western District of Kentucky, after the American Bar Association deemed him "not qualified" for the position.

That rating, on top of the fact that most Democrats have opposed many of the president's nominations, has conservative outside groups, such as the Judicial Crisis Network and the Article III Project, preparing for an all-out war in defense of Walker, according to people with direct knowledge of matter.

"The D.C. Circuit is considered the second highest court in the land, and barring a Supreme Court vacancy, this will be the biggest judicial fight this year," Mike Davis, the founder of Article III, told CNBC in an interview on Friday.

Though it's unclear how some of the liberal organizations are planning to take on the Walker nomination, Demand Justice, a group who vehemently opposed Kavanaugh's nomination has already gone on offense.

"The nomination of a Mitch McConnell crony, who has been rated unqualified by his peers, to the second highest court in the country is beyond suspicious," Demand Justice Chief Counsel Christopher Kang said Friday. "We need to know if McConnell contacted Griffith directly, as reports suggest he may have, and if so, what he offered in exchange for his retirement."

Since Democrats are the minority in the Senate, it's unlikely they will be able to block Walker's nomination. Trump and Republicans have repeatedly touted their record of confirming judges across the country.

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Still, the groups want to boost Walker's credibility and defend him against liberal attacks.

During the Kavanaugh hearings, the JCN pledged to spend up to $10 million in support of his confirmation.

Their efforts for Walker will include organizing pro-Walker TV and radio appearances, op-eds, and, in some cases, advising Republican lawmakers on how best to drive the message as to why Walker should be confirmed, the people with direct knowledge added. They noted one of the goals will be to galvanize a conservative coalition around the judge.

Article III, along with close to 20 other conservative groups, sent a letter on Friday to Sen. Lindsey Graham, the chairman of the Senate Judiciary Committee, and Sen. Dianne Feinstein, the committee's ranking Democratic member, to insist on Walker's immediate confirmation.

Read the full letter here

"We strongly support President Trump's nomination of Judge Justin Walker to serve on the United States Court of Appeals for the D.C. Circuit, and we urge you to confirm him at the earliest opportunity," the letter says. "Every member of this Committee and every American of goodwill should support Judge Walker's nomination to the D.C. Circuit."

The others who signed the letter include the founder of Turning Point USA, Charlie Kirk and Erick Erickson, a conservative blogger. The letter was organized by the Article III group.

This group of 501(c)(4) organizations, who do not disclose their donors, will highlight Walker's resume, which includes a job as a former clerk for Kavanaugh and for retired Supreme Court Justice Anthony Kennedy. He also has ties to Senate Majority Leader Mitch McConnell, who knows Walker's grandfather.

Davis was previously the chief counsel for nominations under Sen. Chuck Grassley, who was the chairman of the Senate Judiciary Committee, before Graham got the position. Davis helped Grassley and Republicans on the committee push Kavanaugh's nomination to the Senate floor where he was confirmed with a razor-thin majority.

The Judicial Crisis Network wouldn't confirm how they're planning to fight back against opposition to Walker, but Carrie Severino, who leads the group, pointed to how they've handled past nominations when asked about their strategy.

"We set up websites, we rally the forces of the conservative coalition, we have run ads, we often do fact checking and the reading of peoples records," Severino said. "We are going to be tailoring our position based on the kind of response we get."