By Dustin Volz

WASHINGTON (Reuters) - Next month's Senate election in Wisconsin could gain Silicon Valley a key ally in Washington in the high-tech industry's battle against the U.S. government's growing appetite for more access to private data.

Democrat Russ Feingold, 63, the only lawmaker to vote against the USA Patriot Act in 2001, leads incumbent Republican Senator Ron Johnson in the state in opinion polls ahead of the Nov. 8 election.

Johnson, 61, rode a wave of support from conservative Tea Party activists to victory six years ago, sweeping Feingold out of office. But polls this year have consistently shown Feingold ahead, although recent surveys show a tighter race.

Privacy advocates and former Feingold staffers said they expected Feingold, if returned to office, to be sympathetic to the privacy concerns of technology companies and civil liberties groups on issues such as encryption and domestic spying, at a time when many lawmakers are being pressured to confront security threats from Islamic State and other militant groups.

The Feingold campaign did not respond to requests for comment.

Apple Inc <AAPL.O>, Microsoft Corp <MSFT.O> and other tech giants have tussled in recent years with government agencies over how much user data the companies should be forced to retain and share with investigators hunting for criminal suspects or national security threats.

Those tensions grew after former National Security Agency contractor Edward Snowden leaked secrets about U.S. surveillance practices in 2013. They reached a crescendo earlier this year when the FBI tried to force Apple to unlock an iPhone tied to one of the shooters in a San Bernardino, California, attack that killed 14 people.

Chief among the goals of many companies and privacy advocates is reforming a foreign intelligence authority used to justify once-secret broad internet surveillance programs exposed by Snowden that will expire in December 2017 unless Congress reauthorizes them.

Should Feingold return to Capitol Hill, former staffers said he would probably seek a seat on the Senate Intelligence Committee, where he would have privileged access to classified information about government spying.

Feingold's campaign has received far more contributions than Johnson's from donors employed by tech companies including Alphabet Inc's Google <GOOGL.O> and Intel Corp <INTC.O>, a review of U.S. Federal Election Commission records showed.

VOTED AGAINST PATRIOT ACT

Digital privacy activists have long regarded Feingold as an ally and aggressive overseer of the intelligence community, a reputation he burnished as the sole vote against the USA Patriot Act, which was passed after the Sept. 11 attacks, expanding the government's surveillance capabilities.

In a speech from the Senate floor at the time, Feingold raised concerns that one provision would allow the government to "go on a fishing expedition and collect information on virtually anyone."

Leaks from Snowden in 2013 showed the provision Feingold questioned was later secretly interpreted to conduct bulk surveillance on U.S. phone metadata. That program was curtailed by Congress in 2015.

Feingold "was a true leader in fighting indiscriminate mass surveillance of innocent Americans," U.S. Senator Ron Wyden, an Oregon Democrat who also is among congressional skeptics of government spying, said in a statement.

Wisconsin typically leans Democratic during high-turnout presidential election years, a problem for Johnson, who won by nearly 5 points in 2010 running as a small-government outsider.

"It was pretty clear that 2010 was a wave election and there was nothing that (Feingold) could have done to fend off the challenge from Ron Johnson," said Kenneth Mayer, a political science professor at the University of Wisconsin-Madison.

Johnson has attempted to use Feingold's 18-year Senate record to portray him as soft on national security. William Allison, a Johnson campaign spokesman, added that Feingold had been "willing to completely mislead Wisconsinites about his weak record on national security."

(Editing by Kevin Drawbaugh and Peter Cooney)