Sanders spoke to reporters after meeting with President Barack Obama in the Oval Office.
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Sanders did not endorse Clinton. He says he plans to remain in the race through next week's primary in the District of Columbia, and will focus on the need for statehood for the district.
Sanders also said he will do everything in his power to make sure Trump does not become the next president of the United States
Sanders, the runner-up for the Democratic nomination, headed to the White House on Thursday under intense pressure to drop out and clear the way for Clinton. Though he showed signs he understood the end was near - he was laying off about half his team - he vowed to keep fighting for his movement, which Democratic leaders hope will evolve into a new base of support for Clinton.
Obama, who was expected to formally endorse Clinton following his midday meeting with Sanders, has sought to give the Vermont senator the courtesy of exiting the race on his own terms. On "The Tonight Show Starring Jimmy Fallon" on Wednesday night, he praised the Sanders campaign.
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"It was a healthy thing for the Democratic Party to have a contested primary. I thought that Bernie Sanders brought enormous energy and new ideas," Obama said during a taped appearance on "The Tonight Show Starring Jimmy Fallon". "And he pushed the party and challenged them. I thought it made Hillary a better candidate."
Obama planned to use the meeting, requested by Sanders, to discuss how to build on the enthusiasm Sanders brought to the primary and advance issues like income inequality and campaign finance reform that Sanders championed, the White House said. That's a diplomatic way of saying it's time for Sanders to pass the baton to Clinton, who declared victory over Sanders on Tuesday.
Now head to head in the presidential race, Clinton and Trump have one thing in common: Both are working to woo Sanders supporters once his campaign fully sputters. Trump has said he welcomes Sanders' voters "with open arms" while Clinton vowed to reach out proactively to voters who backed her opponent in the Democratic primary.
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"He has said that he's certainly going to do everything he can to defeat Trump," Clinton said of Sanders in an Associated Press interview. "I'm very much looking forward to working with him to do that."
Trump, despite a string of victories this week that reaffirmed his place as the GOP nominee, was still working to convince wary Republicans that he's presidential material. Looking ahead to an upcoming speech attacking Clinton and her husband, Trump tried to turn the page following a dust-up over his comments about a Hispanic judge's ethnicity
That controversy and others before it have led prominent Republicans, including House Speaker Paul Ryan, to open chastise their party's nominee. Yet Trump's dominance in the GOP race was hard to overstate: He now has 1,542 delegates, including 1,447 required by party rules to vote for him at the convention. It takes just 1,237 delegates to win the Republican nomination.
For Sanders, any rationale for staying in the race grew murkier as even some of his staunchest supporters started looking to Clinton. Sen. Jeff Merkley of Oregon, the one Senate Democrat to endorse Sanders, said Clinton was the nominee and offered his congratulations. And Rep. Raul Grijalva, a Sanders backer from Arizona, suggested the time to rally behind Clinton would come next week when the primary season concludes with the final contest in the District of Columbia.
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"Bernie's going to do the right thing," Grijalva said Wednesday on the sidelines of discussions about the official Democratic Party platform.
Sanders, who also planned to meet Thursday with Senate Democratic leader Harry Reid, promised to continue his campaign through Tuesday's contest. But about half his campaign staff was being laid off, two people familiar with the plans said. They spoke on condition of anonymity because they weren't authorized to speak publicly about the layoffs.
The task of persuading Sanders' supporters to fall in line falls largely to Obama, still one of the Democratic Party's most popular figures. Obama's aides have said he's itching to get off the sidelines and take on Trump, but the key question was whether voters who helped elected him twice would follow his lead now that he's not on the ballot.
There was little reason for overconfidence among Democrats, who've never seen that powerful coalition of minorities, young people and women reliably show up for candidates not named Obama.
"It's going to be hard to get African-American turnout as high as Obama got it, and to get youth turnout as high as Obama got it," said Celinda Lake, a Democratic pollster. "We have to work really hard."
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