(Adds details on vote, background on election)
By Patricia Zengerle
WASHINGTON, June 14 (Reuters) - The U.S. Senate voted overwhelmingly on Tuesday to pass a $602 billion defense authorization bill, despite President Barack Obama's threat to veto the annual policy measure over issues including a ban on closing the Guantanamo Bay military prison.
The vote was 85-13, far more than the majority needed to pass the 100-member Senate. Six Republicans and seven members of the Democratic caucus voted against the bill, which authorizes military spending for the fiscal year that begins on Oct. 1.
Obama gave a long list of objections to the versions of the bill passed by the Senate and House of Representatives, including provisions making it more difficult to close the detention center at the Guantanamo Bay naval base in Cuba and the use of funds specially designated for the wars in Afghanistan and Iraq to allow the military to avoid mandatory spending limits.
The Senate's version of the National Defense Authorization Act, or NDAA, now must be reconciled with the one passed by the House last month before it can be sent to Obama. The House bill also faces a veto threat.
If the bill is finished before November, when every member of the House and one-third of the Senate is up for re-election, a veto could prove challenging for Democrats.
Obama's party is trying to keep the White House, and hopes to regain the Senate majority it lost in 2014, if not control of the House. But Democrats perpetually battle accusations from Republicans that they are soft on defense, a fight that could be more difficult if they block the major Pentagon policy bill.
Republican Senator John McCain, who chairs the Armed Services Committee, said he was disappointed the bill did not include a program that would help Afghans who are in danger because they worked for American troops to move to the United States.
"They are the number one target for the Taliban and for ISIS (Islamic State)," McCain said in the Senate after the bill passed without an amendment extending the program to provide Special Immigrant Visas for interpreters and others who worked for U.S. forces.
Democratic Senator Jeanne Shaheen, who had introduced the amendment, called its omission "unconscionable." In a statement, she said, "For many of them, this could be a death sentence."
The bill would also require young women to register for the military draft, as the Department of Defense opens all of the military, including combat roles, to females. A few conservative senators voted against the measure because of that provision.
The bill passed with a compromise on the use of Russian RD-180 rocket engines. Lawmakers eased a ban on their use late last year, worried that it could drive United Launch alliance, a joint venture of Lockheed Martin Corp and Boeing Co , out of business and leave only privately held SpaceX to transport satellites into space.
An amendment from Republican Senator Cory Gardner and Democrat Bill Nelson gives the Pentagon time to develop and test a replacement for the Russian-made engines and limits their use for launches.
McCain, a harsh critic of Moscow, had tried to stop the use of the RD-180 to send a message to Russia's President Vladimir Putin.
The bill also bars military base closures, despite military leaders saying they have more capacity than they need. Closing bases is a thorny issue for many lawmakers, especially in an election year. (Reporting by Patricia Zengerle; editing by Lisa Von Ahn, G Crosse)