President Trump has decided on his nominee for the Supreme Court, a source with knowledge of the search told Fox News, though the name of the pick still is not known.

The finalists are believed to be four federal judges: Brett Kavanaugh, Raymond Kethledge, Amy Coney Barrett and Thomas Hardiman.

Trump said he will announce his choice to fill Justice Anthony Kennedy's seat Monday night.

“I have long heard that the most important decision a U.S. President can make is the selection of a Supreme Court Justice - Will be announced tonight at 9:00 P.M,” the president tweeted.

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Republicans and Democrats are gearing up for a blistering confirmation process, as Republicans hold a slim majority in the Senate and liberals have expressed fear that Trump’s pick could join the conservatives on the court to someday overturn the Roe v. Wade decision.

KAVANAUGH’S ROLE IN OBAMACARE’S SURVIVAL FIERCELY DEBATED BY CONSERVATIVES

In order for a Supreme Court nominee to be confirmed, he or she needs to receive a simple majority of 51 votes.

Senate majority leader Mitch McConnell, R-Ky., in a speech on the Senate floor Monday afternoon, blasted the “scare tactics” from Democrats and called for Trump’s nominee to face a fair evaluation in the Senate.

“Tonight, President Trump will announce his nominee to fill the current Supreme Court vacancy,” McConnell said. “We don't know who he will name but we already know exactly what unfair tactics the nominee will face.”

Speaking on the Senate floor, Democratic leader Chuck Schumer, D-N.Y., blasted the involvement of the conservative Federalist Society’s Leonard Leo, who helped the president compile his list of potential justices.

“Mr. Leonard Leo is the man who assembled Trump's list of potential Supreme Court nominees and no one, no one, has been more dedicated to overturning Roe v. Wade than Leonard Leo,” Schumer said.

The Judicial Crisis Network made clear Monday they consider this fight even more substantial than the one to confirm Justice Neil Gorsuch.

The conservative group is launching a $1.4 million ad buy on national cable and digital, with a particular focus on four states: Alabama, Indiana, North Dakota and West Virginia. The ad is expected to feature an introductory bio on the president’s yet-to-be-named Supreme Court nominee.

The campaign is set to launch “as soon as the nominee is announced,” according to Judicial Crisis Network.

Meanwhile, liberal groups already have pressured moderate senators to reject Trump’s eventual pick.

“This will be a 50-state campaign. Our members are fired up, our phones are ringing off the hook, and we are ready to fight this fight,” NARAL Pro-Choice America President Ilyse Hogue recently told reporters.

Liberal groups are pressuring not only vulnerable Democrats like North Dakota Sen. Heidi Heitkamp, West Virginia Sen. Joe Manchin and Indiana Sen. Joe Donnelly to oppose Trump’s pick, but also moderate Republican Sens. Lisa Murkowski of Alaska and Susan Collins of Maine, who have supported abortion rights.

Donnelly, Heitkamp, Manchin, along with Alabama Democratic Sen. Doug Jones were invited to White House for Trump's Supreme Court announcement, but all declined the invitation, sources told Fox News. California Democratic Sen. Dianne Feinstein, the ranking member on the Senate Judiciary Committee, also declined an invitation.

Sources who talked to Trump on Sunday morning told Fox News that the president's top two choices are Kavanaugh and Hardiman, though a GOP source said late Sunday that Barrett still has a good chance of being the pick.

Hardiman was the runner-up when Trump nominated Neil Gorsuch to replace the late Justice Antonin Scalia last year. He also has a personal connection to the president, having served with Trump's sister on the 3rd U.S. Circuit Court of Appeals in Philadelphia.

Fox News' Brooke Singman, Serafin Gomez, Samuel Chamberlain and Kaitlyn Schallhorn contributed to this report.