WASHINGTON (AP) — A second term for President Barack Obama would allow him to expand his replacement of Republican-appointed majorities with Democratic ones on the nation's appeals courts, the final stop for almost all challenged federal court rulings.

Despite his slow start in nominating judges and Republican delays in Senate confirmations, Obama has still managed to alter the balance of power on four of the nation's 13 circuit courts of appeals. Given a second term, Obama could have the chance to install Democratic majorities on several others.

Fourteen of the 25 appeals court judges nominated by Obama replaced Republican appointees.

The next president, whether it's Obama or a Republican, also has a reasonable shot at transforming the majority on the Supreme Court, because three justices representing the closely divided court's liberal and conservative wings, as well as its center, will turn 80 before the next presidential term ends.

The three justices are Ruth Bader Ginsburg, the leader of the court's liberal wing, conservative Antonin Scalia, and Anthony Kennedy, who leans conservative but on some issues provides a decisive vote for the liberals.

The next high court opening would cause a titanic confirmation fight if it would allow a Republican president to cement conservative control of the court by replacing Ginsburg or if Obama could give Democratic appointees a working majority for the first time in decades by replacing Scalia or Kennedy.

The prospect of such dramatic change on the Supreme Court, along with the justices' strikingly high-profile election-year docket could heighten the judiciary's importance as an election issue, said Curt Levey, who heads the conservative Committee for Justice. The justices will hear arguments on Obama's health care overhaul in March and Arizona's immigration crackdown in April. The court also could soon decide whether to hear a Texas affirmative action case challenging the use of race as a factor in college admissions.

Even one new justice can produce dramatic change. Justice Samuel Alito replaced the more moderate Justice Sandra Day O'Connor and shifted the outcome in cases on abortion, campaign finance and other key issues, even though both were appointed by Republicans.

Openings on the circuit courts of appeals get much less attention, but those courts have the last say in most legal disputes that are appealed in the federal system. Only about 80 cases make it to the Supreme Court every year.

There are still more Republicans than Democrats on the circuit appeals courts and on the entire federal bench. But if Obama merely filled existing vacancies, Democratic appointees would be the majority on the influential court of appeals in Washington, where four current Supreme Court justices once served, and the Atlanta-based 11th U.S. Circuit Court of Appeals. Republicans also maintain their edge on the 10th Circuit in Denver only because two judgeships are empty.

Two other appeals courts on which Republicans have comfortable majorities could shift over the next four years. The Chicago-based 7th Circuit has four judges in their 70s who were chosen by Presidents Ronald Reagan and George H.W. Bush. In the New Orleans-based 5th Circuit, Judge Emilio Garza, a Republican appointee, will take senior status in August, a move that will open a seat while Garza takes a smaller caseload. Two Reagan picks in their 70s remain on the court.

Twelve Reagan appointees now in their 70s remain on circuit appeals courts or, in the case of Scalia and Kennedy, the Supreme Court.

Republican presidents, in recent decades, have been more aggressive than Democrats in filling those seats with younger, more like-minded lawyers.

Many nominees of Presidents Ronald Reagan and George W. Bush were in their early 40s, some even in their 30s, and with reputations as bold conservatives. By contrast, Obama has frustrated some liberal interest groups mainly by favoring older nominees over younger ones who might be the Democratic equivalents of some of the Reagan and Bush picks. Obama's two youngest appeals court nominees, Goodwin Liu and Caitlin Halligan, were stymied by Republican filibusters in the Senate.

The average age of Obama-nominated appeals court judges is more than 55 years old, higher than any president's going back to Jimmy Carter, according to the liberal interest group Alliance for Justice. The age of these judges matters in an era when presidents regularly look to the circuit appeals courts as the pool for Supreme Court candidates. Younger judges have a chance to develop a record that presidents can examine, yet still be young enough to be considered for the high court.

Alito and Justices Stephen Breyer, Sonia Sotomayor and Clarence Thomas all became appellate judges in their early 40s. Chief Justice John Roberts, a Republican appointee, and Justice Elena Kagan, a Democrat, would have been on the appeals court in Washington before their 40th birthdays had senators not blocked their confirmations. Roberts had to wait another decade before becoming an appeals court judge, while Kagan is the only justice who did not serve as an appellate judge.

Obama's picks have yet to surprise anyone with their decisions, said Levey, head of the conservative interest group. "So Obama's liberal critics can rest assured that if he's re-elected, his transformation of the appeals courts will make a big difference in the law."

Party labels do not always foretell a case's outcome. During recent challenges to the Obama administration's health care overhaul, Republican appeals court judges in Cincinnati and Washington cast deciding votes upholding the law, while a Democratic appointee in Atlanta voted to strike down the requirement that most people buy health insurance or pay a penalty.

Still, there is wide agreement that Obama picks have sharply altered the Richmond-based 4th U.S. Circuit Court of Appeals, which had been dominated by conservative, Republican appointees.