PORTSMOUTH, N.H. (Reuters) - Democrat Bernie Sanders endorsed former rival Hillary Clinton for president in a display of party unity on Tuesday, describing her as the best candidate to fix the United States’ problems and beat Republican Donald Trump in the Nov. 8 election. With Clinton nodding in agreement beside him, Sanders put aside their bitter campaign for the Democratic nomination and said she would take up the fight to ease economic inequality, make college more affordable and expand healthcare coverage for all Americans. “This campaign is about the needs of the American people and addressing the very serious crises that we face, and there is no doubt in my mind that, as we head into November, Hillary Clinton is far and away the best candidate to do that,” he told a raucous crowd in Portsmouth, New Hampshire, that included plenty of vocal Sanders supporters. “I intend to do everything I can to make certain she will be the next president of the United States,” the U.S. senator from Vermont said. His endorsement, five weeks after Clinton became the presumptive Democratic presidential nominee, brought the most prominent holdout in the party’s liberal wing into her camp. Sanders threw Clinton his support less than two weeks before the Democratic National Convention in Philadelphia, where she is to be formally nominated. “I can’t help but reflect on how much more enjoyable this election will be now that we are on the same side,” Clinton said of Sanders. “Thank you, Bernie, for your endorsement, but more than that, thank you for your lifetime of fighting injustice.” Trump ridiculed the move in a series of Twitter posts, saying Sanders had abandoned the supporters who flocked to his insurgent campaign to rein in Wall Street and get big money out of politics. “Bernie Sanders endorsing Crooked Hillary Clinton is like Occupy Wall Street endorsing Goldman Sachs,” Trump said on Twitter.  In a statement, the Trump campaign said Sanders was now officially part of the rigged system the senator had criticized during his long primary battle with Clinton. “Bernie’s endorsement becomes Exhibit A in our rigged system - the Democrat Party is disenfranchising its voters to benefit the select and privileged few,” said Stephen Miller, a senior policy adviser to Trump. In a Reuters/Ipsos opinion poll released on Tuesday, Clinton extended her lead over Trump to 13 percentage points, up from 10 points at the end of last week. The former U.S. secretary of state hopes the Portsmouth appearance with Sanders will help her win over his supporters, some of whom held Sanders signs at the rally and chanted his name. In recent Reuters/Ipsos polling, only about 40 percent of Sanders backers said they would back Clinton, and the crowd at Tuesday’s rally made it clear she still had work to do. “I am absolutely certain I will not vote for Hillary Clinton,” said Gale Bailey, a Sanders supporter and an unemployed graphic designer from Rochester, New Hampshire, who attended the rally in a Sanders T-shirt. “She’s a crook, and I’m not going to vote for a crook,” Bailey said, adding that she would write in Sanders’ name on the November ballot. The joint appearance concluded weeks of negotiations between the two camps as Sanders pressed for concessions from Clinton on his liberal policy agenda. It came after Clinton last week adopted elements of Sanders’ plans for free in-state college tuition and expanded affordable healthcare coverage. Sanders also successfully pushed to include an array of liberal policy positions in the Democratic platform, which a committee approved on Saturday. Sanders did not win all of his policy fights, most notably failing to win support for blocking a congressional vote on the Trans-Pacific Partnership trade deal. But he told the Portsmouth rally that “our job now is to see that platform implemented by a Democratically controlled Senate, a Democratically controlled House and a Hillary Clinton presidency – and I am going to do everything I can to make that happen.” In an email to supporters after the rally, Sanders said he would still push for changes to the party’s nominating process to make it more inclusive. In the months ahead, he will campaign around the country for progressive congressional candidates, he said. Also on Tuesday, a source with knowledge of the process said the Clinton campaign is vetting James Stavridis, a retired four-star Navy admiral, as a potential vice presidential running mate. Stavridis, who served as supreme allied commander at NATO, is currently dean of the Fletcher School of Law and Diplomacy at Tufts University.  Top Democrats, including President Barack Obama and Senator Elizabeth Warren of Massachusetts, a favorite of the party’s liberal wing, have already announced their support for Clinton, leaving Sanders at risk of being left behind in the Democratic battle against Trump. “I think all signs point to the fact that we’re going to have a very united party going into Philadelphia,” Clinton campaign spokesman Brian Fallon said on CNN earlier on Tuesday, “and when you compare it to the Republicans, we’re going to be miles ahead of them.”  Trump has struggled to unify the Republican Party after alienating many establishment figures with his stances on immigration, Muslims and women. A number of prominent Republicans are skipping the party’s convention in Cleveland next week. 