EXCLUSIVE: A coalition of conservative advocates sent a letter to GOP Sen. Susan Collins of Maine urging her to vote against confirming Judge Ketanji Brown Jackson as Supreme Court justice. 

The letter obtained by Fox News Digital raises concerns about Judge Jackson's sentencing of child pornography offenders and her "zealous" work defending Guantanamo Bay detainees.

Sen Susan Collins

Sen. Susan Collins (Alex Edelman/Pool/AFP via Getty Images)

"Judge Jackson should not be on the Supreme Court," the letter states. "We urge you to reject her nomination, as her views on crime and terrorism are deeply disturbing, and outside of the mainstream of judicial philosophy and legal guidelines."

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Jackson defended her work as public defender and her sentences as a federal trial judge, while Democrats have painted the "soft on crime" attacks as unfair and inaccurate ⁠— especially since she has the endorsement of police groups, including the Fraternal Order of Police. 

photo of Ketanji Brown Jackson

Supreme Court nominee Ketanji Brown Jackson testifies during her Senate confirmation hearing on Capitol Hill, Wednesday, March 23, 2022.  (AP Photo/Alex Brandon)

Collins was one of three GOP senators who previously voted to confirm Jackson last year to the U.S. Court of Appeals for the District of Columbia Circuit. The other two were GOP Sens. Lindsey Graham of South Carolina and Lisa Murkowski of Alaska.

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Collins said she has been carefully reviewing the Senate Judiciary Committee hearings and related materials and will make a final decision after that. 

Collins told Fox News Digital Monday that she had concerns about how little Jackson's judicial philosophy was defined in a meeting between the pair earlier this month. On Wednesday, she said there had been progress during the first two days of Senate Judiciary Committee hearings, but she still had some questions. 

Ketanji Brown Jackson Cory Booker

Supreme Court nominee Judge Ketanji Brown Jackson greets Sen. Cory Booker. (AP Photo/Jacquelyn Martin)

"It was more defined than in her meeting with me, but I still haven't viewed all of the hearings, and obviously there was lots of —  there are a lot of issues," Collins said.

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The letter was signed by 13 people: Terry Schilling; president American Principles Project; Jon Schweppe, director of policy and government affairs at the American Principles Project; Aiden Buzzetti, president of Bull Moose Project; Ziven Havens, managing director of Bull Moose Project; L. Brent Bozell III, chairman of ForAmerica; Christian Robey, vice president of external affairs at the Media Research Center; Kristan Hawkins, president of Students for Life of America & Students for Life Action; Jessie Jane Duff, retired U.S. Marine Corps gunnery sergeant and ambassador at America First Policy Institute; Alex Bruesewitz, CEO of X Strategies; Brigitte Gabriel, founder and chairman of ACT for America; Jack Posobiec, senior editor at Human Events; Gavin Wax, president of the New York Young Republican Club; and Josh Hammer, opinion editor at Newsweek.

Fox News' Kelly Laco and Tyler Olson contributed to this report.