One of the leading liberal groups behind the campaign against Supreme Court nominee Neil Gorsuch refused to disclose its donors.

The Constitutional Responsibility Project (CRP) was created by several leading figures in President Obama's re-election campaign in March 2016 to boost the nomination of Washington D.C. appeals court Judge Merrick Garland. After the group failed to pressure Senate Republicans to confirm Garland, it set its sights on Gorsuch, a judge in Colorado's 10th Circuit Court of Appeals.

The group has produced several ads attacking the nomination and the Trump administration as part of a reported "seven-figure ad campaign against Gorsuch" airing nationwide with additional airtime purchased in Arizona and Nevada.

"We need a justice that protects everyone, not just the wealthy or powerful," one ad says.

The CRP, a 501c4 group, has also echoed Democratic critiques on Gorsuch's "troubling views on big money in politics." The group, however, fails to disclose the identities of its donors behind the multi-million dollar effort to shoot down the nomination. CRP spokesman Josh Dorner, a vice president at liberal public affairs giant SKD Knickerbocker, did not return request for comment about whether CRP will disclose its donors.

Democrats have made dark money a centerpiece of their opposition to Gorsuch as Senate Minority Leader Chuck Schumer gears up for an attempted filibuster of the nomination, which would be the first in Supreme Court history. Democrats on the Judiciary Committee called upon Heather McGhee to testify against the nomination during his confirmation hearings. McGhee, the president of non-profit Demos, said that Gorsuch would lead to "big money corrupting our politics completely," despite the fact that her group does not reveal the identity of its largest donors.

Conservative groups are spending big money in an effort to woo Democrats in red states to back the nomination. The Judicial Crisis Network is spending more than $10 million on pro-Gorsuch ads and grassroots efforts with state and national conservative organizations. Sen. Sheldon Whitehouse (D., R.I.) penned a letter Monday calling on Gorsuch to disclose the identities of JCN donors.

"Because the organization is not required to disclose the identities of its donors, the American people have no idea who is so aggressively advocating for your confirmation," the letter says. "Will you ask JCN to disclose the identities of its donors in order to put to rest any concerns that donations made in secret could affect your rulings?"

JCN President Carrie Severino, a former Supreme Court clerk, said that her organization wants to protect the privacy of its donors and does not plan to disclose their identities. She said in a statement that Senate Democrats "are behaving like third-world strongmen whose approach to public policy is to bully and intimidate people with whom they disagree."

"We are proud to count on supporters who value privacy and freedom of expression more than these Senators, who are glorified thugs," she said. "We will continue to support the confirmation of exceptionally qualified nominees like Neil Gorsuch, who enjoy a broad base of bipartisan support because of their commitment to putting the law ahead of any political agenda."

Whitehouse's staff did not respond when asked if he would call upon the Constitutional Responsibility Project or any other liberal dark money group to disclose its donors.