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.

Just hours before President Donald Trump will make his announcement of who will fill the vacancy in the U.S. Supreme Court, Sen. Bob Casey, D- Pa., has already expressed his disapproval of Trump's decision.

“In a nation with over 700 sitting federal judges, many of whom were appointed by Republican presidents, it is outrageous that President Trump will nominate from a list of just 25 dictated to him by the Heritage Foundation,” Casey said in a statement Monday. “This list is the bidding of corporate special interests hell-bent on handing health care over to insurance companies, crushing unions that represent working men and women, and promoting policies that will leave the middle-class further behind. Any judge on this list is fruit of a corrupt process straight from the D.C. swamp.”

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.