Sen. Bernie Sanders endorsed Hillary Clinton and said that he will do "everything I can to make certain" she wins the 2016 presidential election.

"Secretary Clinton has won the Democratic nominating process and I congratulate her for that," Sanders said, after being interrupted by applause.

"I have come here to make it as clear as possible as to why I am endorsing Hillary Clinton and why she must become our next president," Sanders said.

Today's event marks their first joint general election campaign event.

Sanders started his speech by thanking the millions of Americans who voted for him during the Democratic primary and the small donations he received to fund his campaign.

"Together we have begun a political revolution to transform America and that revolution continues!" he said with Clinton at his side. "Together we will continue to fight for a government that represents all of us and not just the one percent.

Clinton became the Democratic Party’s presumptive nominee last month, and while Sanders previously said he would vote her in November, he had withheld his formal endorsement.

The candidates’ march toward unity began last month when Sanders and Clinton met in person in Washington, D.C., to discuss their agendas and the future of the presidential race.

According to sources from both campaigns, Sanders’ campaign manager Jeff Weaver and Clinton campaign manager Robby Mook have remained in daily contact, negotiating policy roll-outs and next steps. Last week, the Clinton campaign unveiled two significant updates to her policy platforms on college affordability and health care, incorporating and crediting many of Sanders’ ideas.

Sources from the Clinton campaign say her team has been conducting outreach to Sanders supporters as well, especially in states that Sanders won during the primary, including New Hampshire.