By Gabriel Debenedetti

WASHINGTON, Oct 24 (Reuters) - Billionaire financier George Soros, a major Democratic donor, is backing an effort to persuade former U.S. Secretary of State Hillary Clinton to run for president in 2016, a spokesman said on Thursday.

Soros, 83, has pledged $25,000 to political action committee Ready For Hillary, the largest and best-funded independent group backing a potential Clinton candidacy. The wife of former President Bill Clinton would be widely viewed as the favorite for the Democratic presidential nomination if she decides to run.

"His support for Ready for Hillary is an extension of his long-held belief in the power of grassroots organizing," said Soros spokesman Michael Vachon.

Soros' pledge puts him on the PAC's National Finance Council, along with several other major Democratic donors and officials, the group said.

But symbolically, Soros' support could mean much more - particularly at a time when Clinton's supporters are seeking to ward off potential challengers to her in Democratic presidential primaries.

Soros was a huge financial backer of Democrats during the 2004 election cycle, donating an estimated $27.5 million. He also contributed heavily to Senator Barack Obama's presidential campaign in 2008. Clinton lost the Democratic nomination to Obama that year.

During last year's election, however, Soros largely stayed on the sidelines, expressing a distaste for new election laws that allowed unlimited fundraising and spending by new "Super PACs" and non-profit groups. Soros eventually supported some independent Democratic groups during the November election.

Clinton, 65, a former U.S. senator from New York whose four-year run as secretary of state ended in February, has acknowledged that she is considering another run for president but has said she likely will not decide until next year.

In the meantime, she has begun to look like a candidate - staying in the spotlight by appearing at a series of paid speeches and fundraisers. Last weekend she publicly endorsed family friend Terry McAuliffe, the Democratic nominee for Virginia governor.

Ready For Hillary, which is not officially affiliated with Clinton, was founded this year by a former aide in Clinton's unsuccessful campaign for the Democratic presidential nomination in 2008.

The group had raised $1.25 million by July and now claims to have more than 20,000 donors, including many longtime Democratic fundraisers and Obama supporters. Members of Obama's 2012 re-election campaign also have signed on.

Besides Soros, Democratic donors Amber and Steve Mostyn and Clinton friend Susie Tompkins Buell are among the major donors to Ready For Hillary. Democratic lawmakers such as U.S. Senators Claire McCaskill of Missouri and Kirsten Gillibrand of New York have publicly urged Clinton to run for president.

The most recent Reuters/Ipsos tracking poll in mid-September showed Clinton nearly 40 percentage points ahead of her nearest potential challenger in a Democratic presidential campaign, Vice President Joe Biden.

Reuters/Ipsos polling also showed her to be Americans' top choice for president, with New Jersey Governor Chris Christie as her closet potential challenger among Republicans. Americans preferred Clinton over Christie by 19 points, the September poll said. (Editing by David Lindsey and Doina Chiacu)