The issue:

With four justices in their seventies, odds are good that whoever is elected president in November will have a chance to fill at least one Supreme Court seat. The next justice could dramatically alter the direction of a court closely divided between conservatives and liberals. One new face on the bench could mean a sea change in how millions get health care, shape the rights of gay Americans and much more.

Where they stand:

President Barack Obama already has put his stamp on the high court by appointing liberal-leaning Justices Elena Kagan and Sonia Sotomayor, 50-somethings who could easily serve a quarter-century or more. Republican nominee Mitt Romney has promised to name justices in the mode of the court's conservatives.

Why it matters:

Since the New Deal, Supreme Court decisions have made huge everyday differences in American lives, from seminal decisions to uphold Social Security, minimum wage laws and other Depression-era reforms to ringing endorsements of equal rights. And anything is possible with five votes, a bare majority of the nine-justice court. Decisions on many of the hot-button issues in recent years have been by 5-4 votes. These include upholding Obama's health care overhaul, favoring gun rights, limiting abortion, striking down campaign finance laws, allowing consideration of race in higher education and erecting barriers to class-action lawsuits.

Supreme Court vacancies always are a big deal. But the stakes become enormous when the president has a chance to put a like-minded justice on the court to take the place of an ideological opponent. Such a switch can change the outcome of some of the court's most important cases. The most recent example of what the change in a single seat can mean was President George W. Bush's selection of Samuel Alito to take the place of Sandra Day O'Connor. Both justices were appointed by Republicans, but Alito is far more conservative than O'Connor on such key issues as abortion, affirmative action and campaign finance.

As things stand now, Anthony Kennedy, 76, is the only justice who leans conservative but sometimes sides with the liberals on an otherwise evenly divided court. The others older than 70 are the liberal-leaning Stephen Breyer, 74, and Ruth Bader Ginsburg, 79, and the conservative stalwart Antonin Scalia, 76. No one has indicated any intention to retire soon, although Obama's re-election could tempt Breyer and Ginsburg to reconsider. Romney's election could prompt Kennedy and Scalia to change their plans since justices, at least recently, tend to retire when their replacement is likely to be of similar ideology.

But what might happen if the next president had an unexpected opportunity to change the court's direction?

Obama has voiced his disagreement with the Citizens United decision in 2010 that has contributed to ever-freer campaign spending. Of his two appointees, Sotomayor was on the losing side of the Citizens United case while Kagan argued the case for the administration in her previous job. On health care, both justices voted to uphold Obama's health care law.

Romney already has called on the court to overturn the Roe v. Wade decision from 1973 that first established a woman's right to an abortion. Romney has said he would appoint justices like Alito, Scalia, Chief Justice John Roberts and Justice Clarence Thomas. He described them as men who follow the text of the Constitution, not their "personal policy preferences."

But the health care case is a reminder that justices who generally vote a certain way do not always vote in a predictable fashion. Roberts, after all, was the decisive — and lone conservative — vote to uphold the health care law.