* Sustain roadblock against fourth Obama appeals court pick

* Democrats threaten rule change to confirm such nominees

* Unclear if Democrats will pull the trigger

By Thomas Ferraro

WASHINGTON, Nov 18 (Reuters) - U.S Senate Republicans on Monday blocked a fourth nominee by President Barack Obama to a District of Columbia appeals court, defying a threatened rule change by Democrats to strip them of their ability to stop such picks.

On a nearly party-line vote of 53-38, seven short of the needed 60, Democrats failed to end a Republican procedural hurdle known as a filibuster and move forward on the nomination Robert Wilkins.

Obama wants to elevate Wilkins, a federal district court judge in Washington since 2011 who has received the American Bar Association's highest rating, to the U.S. Circuit Court of Appeals for the District of Columbia.

But Republicans refused to clear the way for the nominee, making the Harvard-educated Wilkins the fourth Obama pick to this court to be the target of a filibuster this year.

Democrats threatened an unprecedented Senate rules change to reduce from 60 to 51 the number of votes needed to end filibusters against judicial and executive-branch nominees.

"After tonight, the talk about changing the cloture rules for judicial nominations will no longer be just talk. There will be action," said Senate Judiciary Committee Chairman Patrick Leahy, a Vermont Democrat.

"We cannot allow this unprecedented, wholesale obstruction to continue without undermining the Senate's role provided in the Constitution and without harming our independent federal judiciary," Leahy said.

In a statement issued by the White House, Obama said, "This obstruction is completely unprecedented. Four of my predecessor's six nominees to the D.C. Circuit were confirmed. Four of my five nominees to this court have been obstructed."

Senior party aides said it remains unclear if Democratic leaders will decide to pull the trigger on a possible rule change, and if rank-and-file members would support it. That is because such a rule change would hurt Democrats once Republicans win back control of the Senate, which could come as early as next year's elections.

"THINK TWICE"

"I think Democrats will think twice about this. I hope they do," said Republican Senator Jeff Flake of Arizona, a member of Leahy's Judiciary Committee.

"Republicans were wise years ago to pull back when we considered it," Flake said. "Democrats have been wise so far. I hope they continue to be."

The D.C. Circuit is considered the country's second most important court, behind only the U.S. Supreme Court. One of 13 circuit courts, the D.C. court handles cases involving federal regulations and the separation of powers between Congress, the president and the courts.