After months of standing on the sidelines, President Obama is getting ready to hit the campaign trail for presumptive Democratic nominee Hillary Clinton – with a formal endorsement coming as early as the end of this week.

On Monday night, The Associated Press reported that its count of delegates shows Clinton had captured the 2,383 needed to secure the Democratic presidential nomination.

The New York Times and other media outlets have reported that Obama has been waiting to “aggressively campaign” for his party’s nominee.  

The president is already scheduled to speak at a Democratic fundraiser Wednesday in New York -- one day after six states hold their nominating contests.

“He has indicated he wants to spend a lot of time on the campaign trail, so when it’s time to do that, we’ll go out guns a blazing,” White House communications director Jennifer Psaki said. “We are actively thinking through how to use the president on the campaign trail — what works for the nominee, what works for him, and how to utilize his strengths and his appeal.”

Obama’s popularity and the power of the presidency could help Democrats claw back some of the media attention that Donald Trump has dominated in recent months.

Just last week, Obama said the presumptive Republican nominee “occupies about 70 percent of the news right now” and lashed out at American culture for being too celebrity and fame-driven.

But before Obama can get to work, the Democratic Party will have to come together – which means getting Bernie Sanders on board.

Sanders, whose message of income inequality and a rigged political system has resonated with younger Democratic voters,  has vowed to continue his fight all the way to the Democratic convention in July.

"We really need to bring a close to this primary process and get on to defeating Donald Trump," said Nancy Worley, a superdelegate who chairs Alabama's Democratic Party and provided one of the last endorsements to put Clinton over the top.

"It's time to stand behind our presumptive candidate," said Michael Brown, one of two superdelegates from the District of Columbia who came forward in the past week to back Clinton before the city's June 14 primary. "We shouldn't be acting like we are undecided when the people of America have spoken."

Clinton touted the news at a Long Beach, Calif., campaign event, saying the campaign is now on the “brink of a historic … unprecedented moment.” But even she stressed that six states are yet to vote on Tuesday and urged supporters to cast their ballots for her in those contests.

The six states to vote Tuesday include New Jersey, North Dakota, New Mexico, Montana, California, and South Dakota.

Campaign manager Robby Mook said in a statement: “This is an important milestone, but there are six states that are voting Tuesday, with millions of people heading to the polls, and Hillary Clinton is working to earn every vote. We look forward to Tuesday night, when Hillary Clinton will clinch not only a win in the popular vote, but also the majority of pledged delegates."

The Sanders campaign rejected the declaration that Clinton had clinched the party nod, citing its longstanding position that the superdelegates should not count until they actually vote at the convention – as they are free to switch sides before then.

"There is nothing to concede," Sanders told KTVU late Monday at a rally in San Francisco. "Secretary Clinton will not have the requisite number of pledged delegates to win the Democratic nomination. She will be dependent on superdelegates. They vote on July 25th so right now our goal right at this moment (is to) do everything we can to win the primary tomorrow."

Sanders spokesman Michael Briggs accused the media of “a rush to judgement" and "ignoring the Democratic National Committee’s clear statement that it is wrong to count the votes of superdelegates before they actually vote at the convention this summer."

"Our job from now until the convention is to convince those superdelegates that Bernie is by far the strongest candidate against Donald Trump," Briggs said.

On Monday, Sanders' supporters expressed disappointment that the calls were made before California's primary and urged the senator to continue on despite the pronouncements. "We're going to keep fighting until the last vote is counted," said Kristen Elliott, a Sanders' supporter from San Francisco who attended the rally.

Said another attendee, Patrick Bryant of San Francisco: "It's what bookies do. They call fights before they're over."

House Minority Leader Nancy Pelosi endorsed Clinton during an interview on “Good Morning America” before her home state’s primary.

"I'm a voter in California and I have voted for Hillary Clinton for president of the United States and proud to endorse her for that position," Pelosi said, though adding "it's not over until it's over."

Clinton’s presumptive victory Monday came nearly eight years to the day after she conceded her first White House campaign to Obama. Back then, she famously noted her inability to "shatter that highest, hardest glass ceiling."

Campaigning this time as the loyal successor to Obama, Clinton fended off a surprisingly strong challenge from Vermont Sen. Sanders. He mobilized millions with a fervently liberal message and his insurgent candidacy revealed a deep level of national frustration with politics-as-usual, even among Democrats who have controlled the White House since 2009.

Clinton outpaced Sanders in winning new superdelegate endorsements even after his string of primary and caucus wins in May. Following the results in Puerto Rico, it is no longer possible for Sanders to reach the 2,383 needed to win the nomination based on the remaining available pledged delegates and uncommitted superdelegates, according to the AP.

Clinton leads Sanders by more than 3 million cast votes, by 291 pledged delegates and by 523 superdelegates. She also won 29 caucuses and primaries compared to his 21 victories.

Echoing the sentiments of California Gov. Jerry Brown, who overcame a decades-long rivalry with the Clinton family to endorse her last week, many superdelegates expressed a desire to close ranks around a nominee who could defeat Trump in November.

Beyond winning over millions of Sanders supporters who vow to remain loyal to the self-described democratic socialist, Clinton faces challenges as she turns toward November, including criticism of her decision to use a private email server run from her New York home while serving as secretary of state. Her deep unpopularity among Republicans has pushed many leery of Trump to nevertheless embrace his campaign.

"This to me is about saving the country and preventing a third progressive, liberal term, which is what a Clinton presidency would do," House Speaker Paul Ryan told the AP last week after he finally endorsed Trump, weeks after the New Yorker clinched the GOP nomination.

Yet Clinton showed no signs of limping into the general election as she approached the milestone, leaving Sanders behind and focusing on lacerating Trump. She said electing the billionaire businessman, who has spent months hitting her and her husband with bitingly personal attacks, would be a "historic mistake."

"He is not just unprepared. He is temperamentally unfit to hold an office that requires knowledge, stability and immense responsibility," Clinton said last week in a speech that was striking in its forcefulness, previewing a brutal five-month general election campaign to come.

The Associated Press contributed to this report.