Trump appeared to be going back and forth between Kavanaugh, the favorite of White House counsel Don McGahn, and Hardiman, whom the president's sister, Maryanne Trump Barry, a former colleague of Hardiman's, has pressed him to choose.
The finalists are believed to be four federal judges: Brett Kavanaugh, Raymond Kethledge, Amy Coney Barrett and Thomas Hardiman.
A source familiar with the process said Trump has made his choice.
They're all younger than 55 and conservative enough to make a first cut.
The announcement will be made at 9 p.m.
All of the four states were carried by Mr. Trump in 2016.
Trump was reportedly receptive to the idea, though it's unclear whether that will change his decision. She no longer hears cases. Judge Thomas Hardiman is a USA appeals court judge that The Washington Post reports is a "Second Amendment extremist".
Kavanaugh is a former clerk to Anthony Kennedy, and was the principle author of the Ken Starr report on Bill Clinton's affair with Monica Lewinsky.
Kethledge serves as a judge on the U.S. Court of Appeals for the 6th Circuit in Cincinnati, Ohio.
FILE PHOTO: The U.S. Supreme Court is seen as the court nears the end of its term in Washington, U.S., June 11, 2018.
In many ways, Trump's pick will further empower the conservative backlash against progressive victories of the Obama era, on issues like gay rights, immigration and health care legislation that set a torch under his shock general election campaign.
Current position: Barrett was confirmed by the Senate to a lifetime appointment on the 7th Circuit Court of Appeals this past October.
Before retiring in 2013, Kyl was a member of the Senate Judiciary Committee, which will be the first to consider the nomination.
Each of the candidates has a judicial paper trail and list of past comments that could concern Trump and that will be highlighted by Democrats in a confirmation hearing but also draw conservative support.
On the latter point, Leonard Leo, an executive with the conservative Federalist Society who is advising Trump on his second high court pick, broke with Senate Majority Leader Mitch McConnell. As a result, Democrats have accused Republicans of stealing a Supreme Court seat. The nomination is poised to cement the Supreme Court's conservative majority, vastly reshaping the court for decades to come.
Trump has previously said he wanted "pro-life" justices opposed to abortion rights.
He says he plans on making his decision within the next 24-hours.
Presidents weigh all sorts of considerations in deciding on a Supreme Court nominee, often beginning with the big question: Will the choice be confirmed by the Senate?
The new justice can be expected to cast crucial votes on other matters of national importance including gay rights, gun control, the death penalty and voting rights.