The clash between Alito and Obama has some history behind it.

Four years ago, then-Sen. Obama was one of 42 Democrats who opposed Alito's confirmation. He described Alito as a well-qualified judge, but one who "consistently sides on behalf of the powerful against the powerless or on behalf of a corporation against upholding Americans' individual rights."

A year later, Alito wrote the court's 5-4 decision in the Lilly Ledbetter case, which threw out a sex-bias verdict in favor of an Alabama woman. Obama repeatedly criticized the ruling during his campaign, and as president signed a law to undo it.

Last year, a few days before Obama's inauguration, he and Vice President-elect Joe Biden visited the Supreme Court at the invitation of Chief Justice John G. Roberts Jr. All of the justices were there for the friendly meeting, with the notable exception of Alito.