Obama and Hillary Clinton

U.S. President Barack Obama formally endorsed Hillary Clinton’s White House bid on Thursday and called for the Democratic Party to unite behind her.

But Clinton’s challenger, Senator Bernie Sanders said although he would work with her to defeat the Republican nominee, Donald Trump, he would stay in the race till 14 June, when the last party primary would take place in Washington DC.

Clinton, the presumptive Democratic nominee, said it “means the world” to her that Obama has her back.

The endorsement increases pressure on Sanders, a U.S. senator from Vermont, to concede the race so the party can focus on campaigning against Donald Trump, the Republican presumptive nominee for the Nov. 8 election.

“It is absolutely a joy and an honour that President Obama and I over the years have gone from fierce competitors to true friends,” Clinton told Reuters in an interview.

(Watch the Obama endorsement video here: )

Obama defeated Clinton in the 2008 Democratic primary and she went on to serve as secretary of state in his first term.

Obama, who enjoys strong approval ratings after nearly eight years in office, will campaign with Clinton next week in Wisconsin, her campaign said.

“I don’t think there’s ever been someone so qualified to hold this office,” Obama said of Clinton in a video. “I’m with her. I am fired up, and I cannot wait to get out there and campaign for Hillary,” Obama said.

Obama had been expected to support Clinton since she won enough delegates this week to clinch the Democratic nomination and become the first woman to lead a major U.S. party as its presidential nominee.

Sanders, who met with Obama at the White House earlier on Thursday, said afterward he would work with Clinton to defeat Trump.

Sanders said, however, that he would stay in the race to compete in the final Democratic primary vote in Washington, D.C., on June 14.

Clinton, the former secretary of state, won enough delegates to secure the Democratic nomination this week and become the first woman to lead a major U.S. party as its presidential nominee.

Sanders said he spoke briefly to Clinton on Tuesday night and congratulated her on her “strong campaign.”

“I look forward to meeting with her in the near future to see how we can work together to defeat Donald Trump and to create a government which represents all of us and not just the 1 percent,” he said.

“It is unbelievable to me – and I say this in all sincerity – that the Republican Party would have a candidate for president who in the year 2016 makes bigotry and discrimination the cornerstone of his campaign,” Sanders said, referring to Trump.

Sanders’ campaign has previously said he would carry his populist campaign to the Democratic National Convention in July, when the party’s nominee is formally chosen.