John Podesta, Bill Clinton's former chief of staff and the presumptive campaign manager of Clinton's campaign, sent an e-mail to former staffers on Hillary Clinton's campaign confirming she is running for president in 2016.
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Podesta wrote that Clinton is "hitting the road to Iowa to start talking directly with voters."
Shortly after Podesta's e-mail went out, an official website with the HillaryClinton.com URL went live with the video formally announcing the launch.
"Everyday Americans need a champion. I want to be that champion," Clinton said in the video.
"Americans have fought their way back from tough economic times, but the deck is still stacked in favor of those at the top," she said.
After the video went live, Clinton again confirmed her candidacy in a Tweet.
I'm running for president. Everyday Americans need a champion, and I want to be that champion. –H https://t.co/w8Hoe1pbtC
Clinton's Republican counterparts wasted no time slamming her candidacy, immediately slamming Clinton on their respective social media accounts.
"Hillary Clinton represents the failed policies of the past," Sen. Ted Cruz said in a video posted to his Twitter account. "Does America want a third Obama term, or are we ready for strong, Conservative leadership to make America great?"
Former Florida Governor Jeb Bush echoed Cruz's comments, tweeting that "we must do better than Hillary."
We must do better than Hillary. If you're committed to stopping her, add your name now. https://t.co/GUtxMw19Oh
President Barack Obama said Sunday that he thinks Hillary Rodham Clinton would be "an excellent president."
Obama called Clinton a formidable candidate against him when they competed for the Democratic nomination in 2008, adding that she became a great supporter of his in the general election that year, and that she was an outstanding secretary of state during his first term.
Obama also said that Clinton will have strong messages to deliver if she does decide to run.
Republican presidential candidate Senator Rand Paul said Hillary Clinton's time as secretary of state cast doubt on her qualifications to be president, joining what promised to be a heavy day of Republican criticism as she launches her White House bid.
In her previous presidential run in 2008, Clinton lost to the current president, Barack Obama, in part because of the Obama campaign's clever use of social media to draw attention to his candidacy and raise huge sums through small donations.
Paul repeated both longstanding criticism of her handling of a 2012 attack on a U.S. diplomatic compound in Benghazi, Libya, and of recent questions about foreign contributions to a charity established by her and former President Bill Clinton.
"There is a history of the Clintons feeling they are above the law," the Kentucky senator, who recently announced his presidential bid, told CNN's "State of the Union" program.
The Clinton Foundation's acceptance of contributions from countries like Saudi Arabia, criticized for its strict dress and other rules that discriminate against women, "makes it difficult for her message to appear sincere," Paul said.
Referring to what some Republicans feel was an inadequate security response to threats against the Benghazi consulate, Paul questioned, "would she be there for the 3 a.m. phone call?" as commander-in-chief.
Her campaign is expected to concentrate on making the 67-year-old former first lady relatable to ordinary Americans. Clinton spent four years jetting to foreign capitals as Obama's first-term secretary of state but has had limited day-to-day contact with everyday Americans.
Clinton, the wife of former President Bill Clinton, is the overwhelming favorite for the Democratic nomination and no other major Democratic figure has stepped forward to challenge her, although former Maryland Governor Martin O'Malley has made moves toward a possible campaign.
But Clinton's biggest obstacle may be overcoming her own image.
She has struggled to get past accusations that she might be too secretive based on the revelation earlier this year that she had gone against federal recommendations to use an official email account while at the State Department and instead used her own private server.
A Quinnipiac University poll released this week had the surprising result that she was even or only slightly ahead of some Republican contenders like libertarian Senator Rand Paul of Kentucky. Most polls in recent months have shown her with a substantial lead over the Republican 2016 field.
AP and Reuters contributed to this report.