By Emily Stephenson

WASHINGTON (Reuters) - White House contender Hillary Clinton heads into Thursday's congressional hearing on the 2012 Benghazi attacks with most U.S. Democrats agreeing with her accusation that the Republican-led probe is politically motivated, according to a Reuters/Ipsos poll.

Fifty-four percent of the 598 Democrats surveyed said they believed the U.S. House of Representatives investigation into the attacks and Clinton's actions was entirely or mostly about discrediting the former U.S. secretary of state.

Twelve percent of Democrats surveyed online from Oct. 16-19 said the hearings were mostly a valid investigation that had become politicized. Five percent said the probe, which uncovered Clinton's use of a private email account and a private email server while she ran the State Department, was completely valid.

Other Democrats said they had either not heard of the issue or did not care about it.

The sense among Democrats that politics motivated the Republican investigation could help neutralize questions about Clinton's emails during the primary process to pick the party's nominee for the November 2016 election.

"I feel like the questions have already been answered, and that this is something that they should have let go of a long time ago," Diana Harper, a 70-year-old Arizona Democrat, said of the probe in a telephone interview.

The 2012 attacks resulted in the deaths of four Americans, including the U.S. ambassador to Libya, Chris Stevens. Clinton testified before Congress about the incident in 2013.

This week, she will appear before a special House panel investigating the incident to answer questions about Benghazi and her use of the private email account while in office.

Republicans on the Benghazi panel say they want to answer questions, not score political points. But other Republicans, most notably No. 2 House Republican Kevin McCarthy, made public comments speculating about how the panel's work affected Clinton's poll numbers.

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Clinton said in an interview with CNN on Friday that she had already answered questions about Benghazi and that the panel had become a "partisan arm" of the Republican Party.

In another recent Reuters poll, she led the Democratic field with about 49 percent. Liberal U.S. Senator Bernie Sanders of Vermont came in second, with 26 percent.

Harper, who is leaning toward voting for Clinton, said she initially thought Clinton's email setup might not have been secure. After the State Department began releasing the emails, Harper said she decided Republicans were trying to "discredit the front-runner."

Clinton could face skepticism in a general election if she is the Democratic nominee, the poll on the Benghazi hearings found. About 40 percent of independents said the hearings were mostly or completely valid.

"Is it political? Well yeah, it's Congress; it's always political," Richard Witt, a 58-year-old independent from Omaha, Nebraska, said in a phone interview. "But that doesn't belie the fact that there are legitimate issues that are being investigated."

Witt, who is retired from the military, said he had concerns about how the State Department handled classified information on Clinton's watch.

The poll included 1,517 Americans, including the 598 Democrats, and had a credibility interval, a measure of accuracy, of plus or minus 2.9 percentage points.

Participants from all parties were split evenly on whether the issue would hurt Clinton. Fifty-seven percent of Democrats said it would not, while 43 percent said it would.

The poll showed little danger to Republicans pursuing the probe. Eighty-six percent of respondents said the hearings would not hurt the party, while only 14 percent said it would.

(Reporting by Emily Stephenson; Editing by Jonathan Oatis)