WASHINGTON (Reuters) - Barack Obama is close to becoming the first president in at least half a century to finish a full term without making an appointment to a U.S. appeals court, considered second in importance only to the Supreme Court.

When the U.S. Senate returns next week, a new chapter in the fight over judicial nominations will begin, with the stakes especially high for the Washington, D.C.-based court that hears challenges to government regulations, including those on environmental law and civil rights. The D.C. Circuit, as it is called, is also often a springboard to the Supreme Court where four of the current nine justices served on the D.C. Circuit.

Obama's failure to put anyone on the 11-judge D.C. Circuit, where three vacancies now exist, reflects both rising partisanship and Obama's early priorities.

Partisan rancor has been a factor since Obama, a Democrat, took office in January 2009, and has increased since Republicans gained control of the House of Representativesand raised their numbers in the Senate in January 2011.

In the early months of his administration, Obama was focused on domestic policy related to the financial crisis and a healthcare overhaul.

Senate Republicans blocked the Democratic president's one nominee to the D.C. Circuit in December, and the administration has yet to offer any new candidates.

"It is now getting almost too late for this presidential term, especially in the thick of an election year," said University of North Carolina law professor Michael Gerhardt, who has studied nominations and was special counsel to Senate Judiciary Committee ChairmanPatrick Leahy during the Sonia Sotomayor and Elena Kagan Supreme Court confirmations.

"That would leave the second most important court in the land without the kind of balance he might have achieved," Gerhardt added.

Of the eight active judges on the D.C. Circuit, five are appointees of Republican presidents, three of Democratic presidents. Although the court has 11 members, it routinely hears cases in three-judge panels, assigned randomly to cases, as do other federal appeals courts throughout the country.

"The president will be nominating judges for the D.C. Circuit in short order, but we hope the Senate moves quickly to confirm all of our pending judicial nominees," White Housespokesman Eric Schultz said on Thursday in an email response to questions. He declined to comment on names that might be in the mix.

Two of the three openings on the D.C. Circuit have existed since Obama took office. Obama nominated Caitlin Halligan, a former New York state solicitor general who is now general counsel for the Manhattan District Attorney's office, in September 2010.

Since Senate Republicans prevented a vote on her nomination on December 6, a separate partisan fight has erupted over Obama's recess appointments to two agencies, includingRichard Cordray as head of the Consumer Financial Protection Bureau.

Even without such controversy, the window for Senate confirmation of judicial candidates in an election year begins to close around June, said Sheldon Goldman, a political science professor at the University of Massachusetts, Amherst, who has studied judicial nominations since the early 1960s.

Goldman said Obama could become the only president who was unable to get a single one of his nominees to the D.C. Circuit confirmed.

Federal Judicial Center data also show that for more than 50 years no previous president went a full four-year term without making a single appointment to the D.C. Circuit.

TUSSLING FOR YEARS

Democrats and Republicans have long wrangled over appointments to the D.C. Circuit. During George W. Bush's presidency, for example, Democrats repeatedly blocked Washington lawyer Miguel Estrada, whom Bush named in May 2001; Estrada withdrew in September 2003. Bush, however, named four judges over his two terms, John Roberts (now chief justice of the United States), Janice Rogers Brown, Thomas B. Griffith and Brett M. Kavanaugh.

During President Bill Clinton's second term, Senate Republicans stalled Kagan, then a top Clinton policy adviser, for the D.C. Circuit. Yet Clinton successfully appointed David S. Tatel, Judith W. Rogers, and Merrick B. Garland.

Halligan, nominated by Obama nearly two years into his term, received the American Bar Association's highest rating. Senators who blocked her vote, including Iowa SenatorCharles Grassley, ranking Republican on the Senate Judiciary Committee, criticized Halligan's work as New York's top appellate lawyer in litigation against gun manufacturers. Democratic senators who backed her, including New York DemocratCharles Schumer, stressed that the litigation was spearheaded by then New York attorney general Eliot Spitzer.

A total of 85 vacancies exist in the 874-seat federal judiciary, according to the Administrative Office of the U.S. Courts. Of those, 33 vacancies have been declared "emergencies," based on case backlog and the length of an opening.

"The Obama administration got off to a slow start but is now up to speed compared to the previous administrations," Goldman said. "But the rate and pace of Senate confirmations is now much slower than previous administrations. The Republicans in the Senate are engaged in unprecedented delays."

Activists on the right and left who closely follow judges are waiting to see what the White House does next.

Curt Levey, executive director of the Washington-based Committee for Justice and a critic of Halligan, said Republican senators should particularly scrutinize any new nominee "because of what happened under the Bush administration" with Estrada. "The chance of any D.C. Circuit nominee getting through now is rather small," he added. "I hope it doesn't happen, until there's a deal that includes somebody who President Bush wanted on the court but wasn't confirmed for political reasons."

Nan Aron, president of the liberal Alliance for Justice, said, "We'd be extremely disappointed if at the end of the president's first term, he wasn't well on his way to fully staffing this important circuit. We have every confidence that the president understands what's at stake."

(Additional reporting by David Ingram; Editing by Howard Goller and Sandra Maler)