Pennsylvania Sen. Bob Casey expresses disapproval of Supreme Court nominee before final decision

"Any judge on this list is fruit of a corrupt process..." he said.

Casey also took a hit at the Heritage Foundation, calling it an “extreme Right organization,” claiming the group strives to cut Medicaid and accused Heritage of calling labor unions "cartels."

The six-year incumbent will face Republican nominee Lou Barletta in November to keep his seat. On Monday, Barletta said in a tweet that Casey was “blinded by partisanship” to have opposed a nominee before the pick is announced.

White House Principal Deputy Press Secretary Raj Shah also weighed in on Casey’s statement, tweeting that the senator's opposition comes as no surprise.

Trump’s nominee will replace Associate Justice Anthony Kennedy who will retire later this month. Multiple sources have told ABC News that Trump has narrowed his decision to four potential candidates. Among these individuals are Amy Coney Barrett, Thomas Hardiman, Brett Kavanaugh, and Raymond Kethledge. A final decision is set to be announced by Monday at 9 p.m. ET.