WASHINGTON - US Senator and Democratic presidential candidate Bernie Sanders has officially endorsed his rival Hillary Clinton at a joint rally in a show of party unity.

"Secretary Clinton has won the Democratic nominating process, and I congratulate her for that,” Sanders said on Tuesday at a rally in New Hampshire, a state in the northeastern United States.

"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 Vermont Senator added.

Sanders, 74, an a self-described socialist, told the cheering crowd that Clinton should win the battle against Republican’s candidate Donald Trump and “become our next president.”

"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. Experts believe that Sanders’ decision to support the former secretary of state could empower Clinton against Trump in the presidential race and help her secure liberal votes. Sanders had earlier said that he would not endorse Clinton for president until they meet and he could measure her commitment to combating wealth inequality, and other issues that powered his presidential campaign. Sanders’ emphasis on US income inequality and the influence of corporate money on elections and the government helped him attract millions of voters to his campaign.

Trump, who has proclaimed himself as “the law and order” candidate, will campaign in Indiana later on Tuesday.

Trump immediately criticized Sanders’ endorsement on Twitter, saying, "I am somewhat surprised that Bernie Sanders was not true to himself and his supporters. They are not happy that he is selling out!" 

AFP adds: US Attorney General Loretta Lynch on Tuesday defended her decision not to take legal action against Hillary Clinton over her use of a private email server.

"The matter was handled like any other matter," Lynch said in testimony before the House Judiciary Committee on Capitol Hill.

Republican lawmakers have been sharply critical of the department's decision not to pursue Clinton, the Democratic Party's presumptive nominee for the presidency.

FBI Director James Comey announced last week at the conclusion of an investigation that the Bureau was not recommending that prosecutors bring charges against the former secretary of state. Lynch accepted this recommendation the following day.

She has denied having been influenced by contact with former President Bill Clinton, who Lynch crossed paths with late last month on the tarmac of an airport in Phoenix, Arizona.

"The former president indicated he wanted to say hello. I agreed to say hello," Lynch said at the hearing, describing what she said was a chance encounter with Clinton, who appointed her to her first post as a federal prosecutor in 1999.

"Nothing of any relationship to the email investigation was discussed, nor were any specific cases or matters before the Department of Justice discussed," Lynch said.

House Republicans did not drop the matter, however.

Two among them on Monday asked the Justice Department to begin an investigation of Clinton on suspicion of perjury, accusing her of having made false statements to lawmakers during testimony in October.

While Comey did not recommend charges, he sharply criticized Clinton's handling of classified information on the private server which she used while serving as secretary of state from 2009 to 2013.

Some of that criticism was echoed by lawmakers on Tuesday.

"Her extreme carelessness suggests she cannot be trusted with the nation's most sensitive secrets if she is nevertheless elected president," said Judiciary Committee Chairman Bob Goodlatte, a Republican.

Democrats, who have attacked Republicans for their tenacity in pursuing the email controversy, have suggested that their motives are rooted in election year politics, with just four months until voters go to the polls.

"If any of my colleagues are not yet convinced, it is because they do not want to be convinced," said John Conyers, the top Democrat on the panel.