PORTSMOUTH, United States — After months of bitter campaigning, Bernie Sanders finally endorsed presumptive Democratic presidential nominee Hillary Clinton on Tuesday, pledging to work tirelessly to help his former rival defeat Donald Trump and win the White House.

"Secretary Clinton has won the Democratic nominating process, and I congratulate her for that," Sanders, 74, 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."

The US senator from Vermont offered voters 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.

Putting aside the acrimony, Clinton thanked Sanders for his endorsement -- even if their body language did not exude warmth and was downright awkward at times.

"I am proud to be fighting alongside you," she said. "We are stronger together."

Pence as Trump’s VP?

Trump, who has proclaimed himself "the law and order candidate" amid rising gun violence, unleashed a barrage of criticism, saying Sanders "abandoned" his grassroots supporters by joining forces with Clinton.

"I want to tell you, a lot of Bernie Sanders people are so upset about it, they are going to be voting for Trump," he said in Westfield, Indiana.

The Republican billionaire campaigned there with Governor Mike Pence, raising speculation that he could pick Indiana’s chief executive as runningmate.

Trump told The New York Times he expected to make an announcement by Friday, three days before the Republican convention in Cleveland, where he will officially become the nominee.

He called Pence a "good man" during his unscripted remarks, but also teased the crowd.

"I don’t know whether he’s going to be your governor or your vice president. Who the hell knows?" Trump said to raucous cheers.

Trump had hit the campaign trail on Monday in Virginia Beach with New Jersey Governor Chris Christie -- one of those on the vice presidential shortlist.

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.

Pence also brings executive experience, and the salt-of-the-earth Midwesterner has shown a steady hand that could help counter the narrative that Trump is too incendiary and quick to provoke.

Pence is a 12-year veteran of Congress, well versed in international affairs from his time on the House Foreign Affairs Committee.

Former House speaker Newt Gingrich, who ran for president in 2012, is also in the runningmate mix.

He said he expected Trump would announce his pick on Wednesday or Thursday. — AFP