A traditional conservative, he advanced gay rights, reinforced abortion rights and erased political spending limits.
Soon after he was sworn into office in 2017, Trump nominated Judge Neil Gorsuch to succeed the late Supreme Court Justice Antonin Scalia, who had died a year earlier.
He sided with the court's four liberal justices on same-sex marriage: He wrote several rulings expanding marriage rights, including the 2015 decision that legalized same-sex marriage throughout the country. "There's no reason to consider a replacement on the Supreme Court in that time".
Abortion is likely to be one of the flash points in the nomination fight.
Republicans now hold a bare 51-49 majority in the Senate, although that includes the ailing Sen. Trump said his search for a Supreme Court nominee will "begin immediately" and he intends to make his selection from a list of more than 20 reliably conservative candidates the White House released past year.
Later on his own show, Matthews advocated that the Democrats stall any nomination the president might make, just as Senate Majority Leader Mitch McConnell (R-Ky.) did to former President Obama's court nominations.
"I expect the nominee to be like Justice Gorsuch, to demonstrate excellence in every respect, and to earn widespread support from the American people, and bipartisan support for confirmation in the Senate", Leo said.
He was often referred to as the "swing vote" between the four more liberal and four more reliably conservative justices now presiding over the court.
The other two older justices, Ruth Bader Ginsburg, 85, and Stephen Breyer, 79, are Democratic appointees who would not appear to be going anywhere during a Trump administration if they can help it.