U.S. President Barack Obama formally endorsed fellow Democrat Hillary Clinton for president on Thursday, and called for the Democratic Party to unite behind her after a protracted battle with Bernie Sanders for the party nomination.

"I'm with her. I am fired up, and I cannot wait to get out there and campaign for Hillary," Obama said in a video released

by the Clinton campaign.

The endorsement increases pressure on Sanders, a U.S. senator from Vermont, to bow out of the race and lend his

support to Clinton so that the party can focus on defeating Donald Trump, the Republican Party's presumptive nominee for the Nov. 8 election.

After an unexpectedly tough battle against Sanders's challenge from the left, former first lady Clinton made history when she reached the number of delegates needed to essentially win the party nomination this week, with 2,203 delegates secured and another 577 so-called superdelegates pledged to support her. That made her the first woman to lead a major U.S. party as its White House candidate.

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

"I don't think there's ever been someone so qualified to hold this office," Obama said in his endorsement.

Clinton quickly thanked the president.

Honored to have you with me, <a href="https://twitter.com/POTUS">@POTUS</a>. I'm fired up and ready to go! -H —@HillaryClinton

Trump assailed the endorsement on Twitter: "He wants four more years of Obama-but nobody else does!"

Obama just endorsed Crooked Hillary. He wants four more years of Obama—but nobody else does! —@realDonaldTrump

The brash response from the Clinton campaign — "Delete your account." — quickly went viral, getting retweeted more than 270,000 times and receiving more than 300,000 "likes."

Delete your account. <a href="https://t.co/Oa92sncRQY">https://t.co/Oa92sncRQY</a> —@HillaryClinton

Trump has 8.76 million followers on the social media site to Clinton's 6.6 million. "Delete your account" is a common way to dismiss someone on Twitter.

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

"Needless to say, I am going to do everything in my power and I will work as hard as I can to make sure that Donald Trump does not become president of the United States," Sanders said, standing in the White House driveway with his wife, Jane, at his side.

He has been reluctant to concede the race, and told reporters he would compete in the final Democratic primary vote in Washington, D.C., on June 14. He also said he planned to press for his "issues" — rather than victory — at the party's July convention.

Later in the day, Senate Democratic leader Harry Reid emerged from a meeting with Sanders and said they did not discuss a timeline for ending Sanders's presidential campaign, but that the Vermont senator has "accepted" that Clinton is the party's nominee. New York Senator Charles Schumer and Vice-President Joe Biden also met separately with Sanders.