Jonathan Ernst / Reuters

Kennedy played an important role as a "swing-vote justice," meaning his vote provided the 5–4 vote needed for a majority ruling on important cases that divided the court.

Supreme Court justices serve for decades — Kennedy did so for more than 30 years. So Trump's nominee will potentially shape important legal decisions for years to come.

New cases could come up that would allow the court to revisit 5–4 decisions Kennedy was part of, like those related to marriage equality and abortion. Kennedy will step down July 31, and Trump has announced that he will select a candidate from a previous list of 25 possible candidates that was issued in November.

That, combined with the recent Supreme Court decision that said unlicensed pregnancy crisis centers in California did not have to provide information about family planning services to patients, has people concerned about their reproductive rights, including access to birth control and safe abortions.