By Phil Stewart and Idrees Ali
WASHINGTON, Nov 7 (Reuters) - After winning support from scores of prominent Republicans during his campaign, Joe Biden will take office with the possibility of unprecedented bipartisan backing on national security as he moves to restore traditional policy toward America's allies and its adversaries.
But it is unclear how long that backing will endure among conservatives who endorsed the Democrat as the antidote to President Donald Trump's anti-globalist agenda, which clashed with long-held Republican positions on military alliances and raised doubts about the United States' role in the world.
"I think Biden may have some wind at his back on this that no president has had before," Chuck Hagel, a former Republican senator who served as defense secretary in President Barack Obama's administration, told Reuters.
Hagel, who had endorsed Biden, said he expected the former vice president and U.S. senator to bring some Republicans into his administration and govern in a way that builds consensus.
Biden secured enough Electoral College votes to clinch the presidency on Saturday and major allies congratulated him on his victory.
But strains between Republicans and Democrats were already building in the run-up to the Nov. 3 presidential election. Experts say Biden's ability to sustain bipartisan support into 2021 may depend on how he handles pressure within the Democratic Party to rein in military spending and how he confronts American rivals, including Russia, China and Iran.
Adam Smith, the Democrat who leads the U.S. House of Representatives Armed Services Committee, acknowledged friction over defense spending. Even Trump's own budget proposal in 2020 had foreseen largely flat spending in a second term in office after increases in his first term.
"There is a growing wing, the progressive wing of our (Democratic) party, that wants to see the defense budget cut on general principle," Smith said before the election.
"They think that spending money on bombs and weapons of war is nowhere near as important spending money on well, just about anything else, for that matter," he added.
Smith himself has questioned Navy plans to build a 500-ship fleet and criticized the Trump administration's guiding strategy document on national defense, which focuses on countering both Russia and China. He recently called it "a recipe for a very dangerous and unnecessary Cold War."
That document has broad support among Republicans.
Still, Mac Thornberry, the top Republican on the committee, told Reuters he was optimistic for the prospect of broad cooperation between Republicans and Democrats under a Biden administration.
He noted that the strongest voices among Democrats for major spending cuts were not major players on national security.
"If you look at some of those people who are the loudest, (they) have never voted for defense anyway," Thornberry said.
'RATIONAL, COHESIVE MIDDLE'
The list of Republican national security experts who endorsed Biden is long and includes Colin Powell, once chairman of the military's Joint Chiefs of Staff and secretary of state under former President George W. Bush, as well as Richard Armitage, a former senior Republican State Department and Pentagon official.
Biden's backers also include former senior military officers, including William McRaven, the retired admiral who oversaw the operation to kill Osama bin Laden as head of Special Operations Command, as well as Paul Selva, who served as vice chairman of the Joint Chiefs of Staff under Trump.
Both Republicans and Democrats have voiced concern over Trump's approach to long-standing U.S. alliances, including his criticism of NATO and his efforts to squeeze more contributions from South Korea to help offset the cost of U.S. troops there.
Susan Rice, who served as White House national security adviser when Biden was vice president, pointed to the retired generals, admirals and former Republicans who backed Biden as evidence that most national security leaders wanted a return to more traditional policies.
"There remains a rational, cohesive middle in the mainstream of national security decision-making and expertise," Rice told Reuters.
"Traditionally, before Trump, foreign policy and national security were largely played within the 40-yard-lines," she said, adding that "the partisan shifts were not extreme."
"They have become extreme."
Jon Alterman, with the Center for Strategic and International Studies think tank, said Biden and many of his close aides were known quantities in Washington, giving them credibility on national security that helps build consensus.
"That gives the president a huge multiplier effect, because it's my view that a Biden administration would be much, much better at marshaling a whole-of-government approach and coordinating the entire government behind the president's intentions," Alterman said. (Reporting by Phil Stewart and Idrees Ali; Editing by Mary Milliken and Peter Cooney)