President Donald Trump plans to announce his Supreme Court pick in a prime-time address on Monday — and four candidates remain in the running.

"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.," Trump tweeted Monday morning.

There are apparently three front-runners remaining to replace retiring Supreme Court Justice Anthony Kennedy, all of whom are appeals court judges: Thomas Hardiman of the 3rd Circuit; Brett Kavanaugh of the District of Columbia Circuit; and Raymond Kethledge of the 6th Circuit. Earlier in the day, Amy Coney Barrett of the 7th Circuit had been on the list, but she was spotted at her home in Indiana Monday evening, meaning that it appeared to be impossible for her to be at the White House for the 9 p.m. event.

The president lately has focused his attention on Kavanaugh and Hardiman, a source told NBC News, although all four remain in the running.

Kavanaugh, 53, is a former law clerk to Kennedy, as was Neil Gorsuch, Trump's first Supreme Court nominee, who succeeded Antonin Scalia. Kavanaugh is also a veteran of George W. Bush’s White House, where he served as a staff secretary. He has called Roe v. Wade, the 1973 landmark Supreme Court case that legalized abortion, a "binding precedent."

Hardiman, 53, was appointed by Bush to serve on the court and was the runner-up to Gorsuch to replace Scalia. Hardiman has defended gun rights but also has ruled in favor of immigrants seeking asylum. He also served with Maryanne Trump Barry, the president's older sister and a federal judge.

Last year, Trump created similar suspense when he announced Gorsuch as his nominee, keeping the announcement under wraps until the last minute. That prime-time announcement garnered more than 30 million viewers.

Trump said that he wants to appoint a justice that could serve on the Supreme Court for decades.

"We have to pick one that's going to be there for 40 years, 45 years," Trump said recently.

The president told reporters on Sunday that he expected to make his final decision by noon on Monday. "I'm getting very close to making a final decision, and I believe this person will do a great job," the president said.

Democrats, who hold a minority in the Senate, have been gearing up for a confirmation battle, arguing that a vote on the nominee should not take place before the midterm elections. Republicans maintain a razor-thin margin in the Senate, 51-49.

Sen. Dick Durbin, D-Ill., told NBC's "Meet the Press" on Sunday that he was not sure if Democrats will be able to stick together on the vote but he expects each Democrat to "look carefully at each nominee." However, several Senate Democrats are facing tough re-election bids in red states Trump handily won in 2016. Three of those Democrats voted to confirm Gorsuch last year.

Sen. Chris Coons, D-Del., told MSNBC's "Morning Joe" on Monday that "there's precious little" Democrats can do to block the president's nominee.

"There's very little we can do with just 49 members that would prevent them from moving ahead with a confirmation," Coons said.

Sen. Roy Blunt, R-Mo., told "Meet the Press" Sunday that the Senate "can confirm any of the four names being mentioned," adding, "The president has to think about who is the easiest to get confirmed here."