Right now, the Supreme Court of the United States is almost evenly balanced. There are four justices that almost always rule on the more conservative side of issues, and there are four justices whose rulings are usually on the more liberal end of the political spectrum. One justice is seen as a swing vote as Justice Kennedy votes on each side depending on the issue.
Because the Supreme Court is so balanced, anytime that there is a vacancy on the court, the political stakes are high. When President Obama wanted to get his nominee Merrick Garland confirmed for the vacancy left upon the death of Justice Scalia, Senator Charles Grassley, who chairs the Senate Judiciary Committee, would not even let the nomination come up for a vote.
According to Reuters, Senator Grassley is causing a political and legal controversy by suggesting that any justice who is thinking about retiring soon do so right now so that the Senate will have time to confirm the appointment before the 2018 mid-term elections.
Observers believe that this is directed straight at Justice Kennedy. If he were to retire right now, President Trump would be able to appoint a conservative and change the balance of the court to one that is overwhelmingly conservative.
At this time, Justice Kennedy has not even hinted that he is thinking about giving up his seat on the court. Most observers do not believe that any other justice is contemplating retirement.
Justice Ginsberg is the oldest member of the court, and she has firmly stated that she has no plans to retire. She would probably not retire unless a more liberal president was in power. Justice Breyer is the next oldest justice, and he has made no announcement that he plans to retire anytime soon either.
If the Republicans were to lose control of the Senate in the mid-term elections, it is possible that Supreme Court justices would not be approved at all. A Democratic Senate is very unlikely to approve anyone nominated to the Supreme Court by President Trump.