Washington: Democratic presidential candidate Bernie Sanders has vowed to prevent Republican Donald Trump from becoming the US president amid concerns about further damage to party unity deriving from his continued stay in the race. "Needless to say, I am going to do everything in my power, and I will work as hard as I can to make sure Trump does not become president of the US," Xinhua quoted Sanders as saying here on Thursday after his meeting with US President Barack Obama. "It is unbelievable to me ... that the Republican Party would have a candidate for president who in 2016 makes bigotry and discrimination the cornerstone of his campaign," he said. Meanwhile, Sanders said he already talked with Hillary Clinton after she amassed enough delegates needed to notch up the nomination on Tuesday. Sanders said he and the former Secretary of State would speak soon about how to "work together" to defeat Trump. However, Sanders still declined to endorse Clinton at the moment and insisted he would take part in the last nomination contest next week. As the Democratic primary season was all but over, the notion of party unity had become a crucial topic in the Democratic field. Despite his mathematical elimination from the race, Sanders had earlier pledged to continue the fight into the national convention in July when party nomination would be formally announced. However, Sanders later also indicated that he would "assess" his path to victory in the wake of California's primary which was held on Tuesday. Clinton notched up an easy victory by two-digit lead in California. Sanders' meeting with Obama in the White House was part of the Vermont senator's busy schedule in Washington on Thursday. He would also meet Democratic leaders in the US Congress and Vice President Joe Biden. Unlike the prompt rapprochement reached between Clinton and Obama in 2008 primary season, reconciliation this time between Sanders and Clinton could be elusive. For one thing, Sanders had for long called himself an independent and democratic socialist, and he joined the Democratic Party only last year to get on the ballot. Therefore, his is less committed to party loyalty than was Clinton eight years ago. Even more daunting a task this time for party establishment to bridge the Clinton and Sanders divide was the anti-establishment sentiment Sanders had stirred up among disheartened Democratic and independent voters in this chaotic primary season. According to the most recent YouGov poll released on May 25, half of Sanders' supporters would turn away from Clinton in a matchup between the former Secretary of State and presumptive Republican nominee Donald Trump. At the end of April, YouGov poll found that 63 percent of Sanders' supporters were willing to vote for Clinton.