PORTSMOUTH: After months of bitter campaigning, Bernie Sanders on Tuesday offered his long-awaited endorsement for Democratic presidential hopeful Hillary Clinton, saying he would work hard to help his former rival win the White House.

The joint appearance at a high school in Portsmouth, New Hampshire was the culmination of weeks of talks between the two campaigns aimed at unifying the party in preparation for taking on Republican Donald Trump in November.

“Secretary Clinton has won the Democratic nominating process, and I congratulate her for that,” Sanders told a cheering crowd, with Clinton at his side.

“She will be the Democratic nominee for president and I intend to do everything I can to make certain she will be the next president of the United States.”

“She must become our next president,” Sanders emphasised, offering a litany of reasons why the 68-year-old former secretary of state is a better choice than the 70-year-old Manhattan real estate mogul.

“If anyone out there thinks that this election is not important, take a moment to think about the Supreme Court justices that Donald Trump will nominate, and what that means to civil liberties, equal rights and the future of our country,” Sanders said.

The 74-year-old Sanders, a US senator from Vermont, waged a tougher-than-expected year-long battle against Clinton, but she clinched enough delegates to secure the nomination in early June.

Sanders, a feisty self-described democratic socialist, nevertheless had refused to concede defeat to his more moderate rival, although he had said he will vote for Clinton.

Sanders wants to ensure that his ideas are part of the party platform presented at the Democratic National Convention later this month in Philadelphia, when Clinton is formally nominated.

Party officials met over the weekend in Orlando, Florida to finalise the Democratic platform, which they described as the most ambitious and progressive in history.

The party reached agreement on language concerning climate change, health care and raising the minimum wage in America to $15 per hour. They reportedly failed, however, to reach common ground on the Trans-Pacific Partnership trade accord.

Trump ready to pick running mate?

Trump, who has proclaimed himself “the law and order candidate” amid rising gun violence, will campaign in Indiana later Tuesday.

His scheduled appearance with Governor Mike Pence is raising speculation that Trump could pick the state's chief executive as his running mate.

“I am the law and order candidate,” the presumptive Republican nominee said Monday in Virginia Beach at an event with New Jersey Governor Chris Christie.

The declaration, reminiscent of remarks made by former president Richard Nixon in his 1968 campaign, came as Trump highlighted recent killings including the horrific shooting deaths of five Dallas police officers in a sniper-like ambush.

“The attack on our Dallas police is an attack on our country. Our whole nation is in mourning,” Trump said, as he pledged to “fight” for law enforcement personnel and ensure they have Washington's full backing.

Trump and Clinton both halted their campaigning for a day after Thursday's Dallas police murders.

Christie, a potential vice presidential pick, joined Trump as reports swirled that the billionaire real estate tycoon will choose a running mate in the coming days.

Christie's experience running a populous state could be seen as critical for Trump, who has acknowledged his own lack of political and government expertise.

Indiana's Pence would bring executive experience as well, along with a perceived steady hand that could help counter the narrative that Trump is too incendiary and quick to provoke.

Pence made a tepid endorsement of Senator Ted Cruz but switched to Trump when Cruz dropped out.

“I'm prepared to make that case anywhere across Indiana and anywhere across this country that Donald Trump would want me to,” he said.