Portsmouth, N.H.

Democrat Bernie Sanders endorsed former rival Hillary Clinton for president in a display of party unity Tuesday, describing her as the best candidate to fix the nation’s problems and beat Republican Donald Trump in the Nov. 8 presidential election.

With Mrs. Clinton nodding in agreement beside him, Mr. 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 health care 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, N.H., that included plenty of vocal Bernie 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 Mrs. Clinton became the presumptive Democratic presidential nominee, brought the most prominent holdout in the party’s liberal wing into her camp. Mr. Sanders threw Mrs. 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,” Mrs. Clinton said of Mr. Sanders. “Thank you, Bernie, for your endorsement, but more than that, thank you for your lifetime of fighting injustice.”

Mr. Trump ridiculed the move in a series of Twitter posts, saying Mr. 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,” Mr. Trump said on Twitter.

In a statement, the Trump campaign said Mr. Sanders was now officially part of the rigged system the senator had criticized during his long primary battle with Mrs. 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 Mr. Trump.

In a Reuters/Ipsos opinion poll released on Tuesday, Mrs. Clinton extended her lead over Mr. Trump to 13 percentage points, up from 10 percentage points at the end of last week.

The former U.S. secretary of state hopes the Portsmouth appearance with Mr. Sanders will help her win over his supporters, some of whom held Bernie Sanders signs at the rally and chanted his name. In recent Reuters/Ipsos polling, only about 40 percent of Mr. Sanders’ backers said they would back Mrs. 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, N.H., who attended the rally in a Sanders T-shirt.

“She’s a crook, and I’m not going to vote for a crook,” Ms. Bailey said, adding that she would write in Mr. Sanders’ name on the November ballot.