Senator Bernie Sanders delivered an impassioned endorsement of Hillary Clinton's presidential candidacy on July 25, telling the Democratic Party’s national convention that his former rival for the nomination "must become the next president of the United States."
On a raucous opening day of the convention that began with Clinton's name booed by Sanders’ supporters, the U.S. senator rallied the crowd behind the former secretary of state as she prepares to battle Republican candidate Donald Trump in the November 8 election.
"Hillary Clinton will make an outstanding president, and I am proud to stand with her tonight," Sanders told some 5,000 party delegates gathered in Philadelphia, where Democratic leaders sought to show a united front against Trump.
That unity appeared elusive for the duration of the evening, with audible boos and jeers -- most likely from Sanders’ fans -- reverberating through the arena as Sanders concluded his speech by throwing his political weight behind Clinton.
Those tensions erupted almost immediately after Baltimore Mayor Stephanie Rawlings-Blake formally opened the four-day convention, which is set to end with Clinton becoming the first woman to be a major U.S. political party's presidential nominee.
Mentions of Clinton's name by speakers on the first day were met with boisterous applause from her supporters, while backers of Sanders booed and chanted "Bernie, Bernie."
"We're all Democrats and we need to act like it," the convention’s chairwoman, U.S. Representative Marcia Fudge of Ohio, shouted over the ruckus as the opening session began.
Sanders, 74, fought a fierce primary battle against Clinton for the Democratic nomination, attracting legions of loyal followers with progressive proposals to reign in Wall Street excesses, diminish the role of money in politics, and make health care and higher education more accessible.
His backers continued their vocal support even after he endorsed Clinton, and the animus of his followers was exacerbated by a stunning e-mail leak before the conference that showed ostensibly neutral Democratic Party officials favoring Clinton in her campaign against Sanders.
After Wikileaks published the more than 19,000 messages on July 22, Democratic National Committee (DNC) chairwoman Debbie Wasserman Schultz resigned and the committee formally apologized to Sanders for the "inexcusable" remarks in the e-mails.
Sanders apparently responded in kind, urging his supporters not to disrupt the convention, and declining to mention the e-mail scandal as he attempted to rouse the crowd for Clinton in his speech to the delegates.
"This election is about which candidate understands the real problems facing this country and has offered real solutions -- not just bombast, not just fear-mongering, not just name-calling and divisiveness," Sanders told the crowd.
Police in Philadelphia, meanwhile, briefly detained more than 50 people on the evening of July 25 after they attempted to hurdle the barricades outside the convention hall out of anger over how party leaders have treated Sanders.
Russian Hacking Claims
Throughout the campaign, Sanders and his supporters had repeatedly complained that the DNC was favoring Clinton in the primary season, accusations that appeared to be substantiated by the trove of e-mails published by Wikileaks on July 22.
The leaked e-mails showed that DNC officials had sought to undercut Sanders’ campaign by raising questions about his Jewish heritage and whether he may be an atheist.
The U.S. Federal Bureau of Investigation said on July 25 that it is investigating "cyber intrusion" by hackers in connection with the e-mail leak, which Clinton's campaign has suggested was carried out by individuals with links to the Russian state.
The Clinton campaign has said the leak may be aimed at helping Trump, who has pledged to seek better ties with Moscow if elected to the White House and has questioned the necessity of assisting fellow NATO members in the Baltics if they were invaded by Russia.
The Trump campaign has ridiculed the suggestion that Russia tried to help him by stealing and leaking the e-mails, calling the claim a "joke." The Kremlin brushed off the claim when asked about it on July 25.
Taking On Trump
Trump, a wealthy businessman and former reality TV star, has faced withering criticism both from Democrats and some of his fellow Republicans for comments they call xenophobic, sexist, and racist.
In at least one national poll on July 25, however, he moved ahead of Clinton after trailing the presumptive Democratic nominee for several months in surveys conducted nationwide.
Trump had a lead of 48 percent compared to 45 percent for Clinton in a two-candidate presidential match-up, according to a CNN/ORC poll.
Trump, who has never held public office, has repeatedly said that Sanders was the victim of a "rigged" system favoring Clinton. On July 25, he accused Sanders of caving to Clinton.
"Bernie Sanders totally sold out to Crooked Hillary Clinton," Trump wrote on Twitter before Sanders had finished his speech.
"All of that work, energy, and money, and nothing to show for it! Waste of time," Trump added.
Earlier in the evening, Trump had needled the opening night of the Democratic convention, writing on Twitter: "Wow, the Republican Convention went so smoothly compared to the Dems total mess."
WATCH: Michelle Obama Urges Democrats To Rally Behind Clinton
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'I'm With Her' -- Michelle Obama Calls For Democrats To Rally Behind Clinton
Sanders spoke shortly after U.S. first lady, Michelle Obama, took the stage in Philadelphia and told the convention that Clinton is the only candidate prepared to be president.
She praised the former secretary of state as the only "person who I believe is truly qualified to be president of the United States" and took a clear swipe at Trump with a denunciation of "hateful language."
"When someone is cruel or acts like a bully, we don't stoop to their level," she said.