By Steve Holland

WASHINGTON (Reuters) - U.S. presidential candidates Hillary Clinton and Donald Trump kicked off a fierce general election battle, with Democrats accusing Trump of erratic behavior and the Republican threatening to bring up old Clinton scandals.

Clinton, the former secretary of state, made history when she became the first woman to lead a major political party in its quest to capture the U.S. presidency. Big primary election wins on Tuesday in California and elsewhere catapulted her to victory over Democratic opponent Bernie Sanders.

If elected on Nov. 8, the 68-year-old former U.S. senator from New York would return the Clinton family to the White House 16 years after her husband, Bill Clinton, completed two terms as president.

All signs point toward a negative campaign as Clinton accused Trump of being temperamentally unfit to serve and the New York businessman charged that Clinton had a dark past and a weak record as President Barack Obama's first-term secretary of state.

Clinton told NBC on Wednesday she would not run a "campaign of insults," but she sought to portray the 69-year-old Trump as not fit for the Oval Office after the real estate developer repeatedly accused a Mexican-American judge of showing bias against him because of his ethnic heritage.

The Clinton campaign has pointed to criticism from leaders in Trump's Republican Party to make this case.

"I'm going to talk about why he's unqualified to be president based on his own words and his deeds. And I'm going to continue to make the case he is temperamentally unfit to be commander in chief," she said in the interview.

Trump gave a carefully crafted primary race victory speech on Tuesday laying out his own plan of attack. To keep from straying off message, he used a Teleprompter and avoided his typical stream-of-consciousness delivery.

Trump said money given to the Clinton Foundation charity from foreign donors had earned the Clintons millions of dollars and had a corrupting influence when Clinton was secretary of state.

"Hillary Clinton turned the State Department into her private hedge fund - the Russians, the Saudis, the Chinese - all gave money to Bill and Hillary and got favorable treatment in return. It's a sad day in America when foreign governments with deep pockets have more influence in our own country than our great citizens," Trump said.

He said he would give a speech next week “discussing all of the things that have taken place with the Clintons.”


A Reuters/Ipsos poll on Tuesday showed Clinton leading Trump by 10 percentage points nationally, little changed from a week earlier. [nL1N18Y1Z2]

Both Clinton and Trump must unite their parties but the Democrat appeared to face the easier path with Sanders, a leftist U.S. senator from Vermont, nearly out of options to challenge her.

Trump has an uphill battle. U.S. House of Representatives Speaker Paul Ryan described Trump's remarks about the judge as a "textbook definition of a racist comment," but said he would still support him.

Trump said on Wednesday he was "disappointed and surprised" by charges of racism from Republicans.

"I had just won more votes than anyone in the history of the party," he told Time. "But you know, they have to say what they have to say. I'm a big boy. They have to say what they have to say."

Ryan met behind closed doors on Wednesday with House Republicans. An aide said Ryan "discussed with his members the thinking behind his endorsement (of Trump) and how to move forward” and reiterated he had confidence Trump would support the House Republican agenda.

"I'd say the general attitude is, 'Good. Now let's just move on,'" said U.S. Representative Chris Collins of New York, a Trump supporter. "The irony here and the frustration was that he's not racist."

Others said Trump needed to stop engaging in petty battles with former rivals and build a fundraising organization. Trump will meet on Thursday in New York with top fundraisers of the Republican National Committee, a party official said.

Hopes flickered among some anti-Trump Republicans that there would be a revolt against him when delegates convene to nominate him formally in Cleveland from July 18 to 21.

Representative Morgan Griffith, a Virginia Republican, said that was unlikely but that Trump needed to improve as a candidate.

"Don't step into the cow pie," Griffith said. "He can't afford to be stepping into any more controversies like that."

Clinton edged Sanders out in a rough-and-tumble battle that stretched over four months and 50 states. She won support, especially among older voters, with a more pragmatic campaign focused on building on the policies of fellow Democrat Obama.

Clinton said she had spoken to Sanders on Tuesday to congratulate him on his campaign.

"I am looking forward to working with him to unify the Democratic Party against the threat that Donald Trump poses to our country," she told PBS on Wednesday. "So we are talking. We will be having an opportunity to discuss in greater detail in the days ahead how we can best work together."

Democratic Party elites are lined up squarely behind Clinton, including most likely Obama, who may endorse her as early as this week. White House Press Secretary Josh Earnest told reporters that Obama would not endorse until after he meets with Sanders on Thursday.

Obama and Sanders have spoken three times in the past month, Earnest said. The president's eventual endorsement would put pressure on Sanders to exit graciously and throw his support to Clinton. Sanders is also due to meet with Senate Minority Leader Harry Reid.

(Additional reporting by Susan Heavey, Doina Chiacu, Emily Stephenson, David Morgan, Roberta Rampton and Richard Cowan in Washington; Editing by Alistair Bell and Peter Cooney)