US Secretary of State Hillary Clinton (L) is joined by her husband former US President Bill Clinton and daughter Chelsea Clinton as she is ceremonially sworn in at the State Department in Washington, in this February 2, 2009 file photo. [Photo/Agencies]

Hillary Rodham Clinton announced her much-awaited second campaign for the White House on Sunday, posting a video declaring that she will focus on greater economic security and opportunity for middle-class and poor Americans.

The former first lady, US senator and secretary of state decided against a big public announcement, opting instead for a two-minute, 18-second video statement on her campaign Facebook page, a low-key approach to a candidacy that has been expected for months.

"Americans have fought their way back from tough economic times. But the deck is still stacked in favor of those at the top. Everyday Americans need a champion and I want to be that champion," Clinton said, speaking only at the end of the video message which features a series of men, women and children describing their aspirations.

Clinton plans to head to the early-voting states of Iowa and New Hampshire, looking to connect with voters directly at coffee shops, day care centers and some private homes. Clinton hopes to avoid making the same stumbles she did in 2008, when she entered the race as a US senator and a heavy favorite only to be upset by Barack Obama in Iowa's lead-off caucuses.

She concluded by saying, "So I'm hitting the road to earn your vote. Because it's your time. And I hope you'll join me on this journey."

This voter-centric approach was picked with a purpose, to show that Clinton is not taking the nomination for granted. Only after about a month of such events will Clinton will give a broader speech outlining more specifics about her rationale for running.

The 67-year-old Clinton brings a long public record to her second bid for the White House, a history that will both help and hurt her candidacy. Republicans were already pushing a message that seeks to attach her to the scandalous upheavals of her husband Bill Clinton's two-term presidency in the 1990s.

What's more, she intends to cast herself as a tenacious fighter determined to block the growing power of an increasingly right-wing Republican party that has sought to block Obama's agenda at every turn and now controls both chambers of Congress..

Obama said on Saturday that he thinks Clinton "would be an excellent president." After Obama defeated Republican Sen. John McCain in 2008, he named her secretary of state, a job she used in an attempt to rebuild US relations with countries around the world that had become critical of the American war in Iraq, which she had voted to authorize as a senator from New York.

Clinton enters the race with polls showing her in a strong position to succeed Obama. However, she will have to overcome history to win election. In the last half-century, the same party has held the White House for three consecutive terms only once, during the administrations of Republicans Ronald Reagan and George H.W. Bush.

Republicans will try to counter Clinton's strong resume by casting her as someone who is not trustworthy. Republicans have jumped on her use of a personal rather than a government email account and a server located in her New York home while she was secretary of state. They have also raised questions about donations from foreign governments to the Clinton family's foundation. She remains under fire from Republicans over her handling of the September 2012 terrorist attacks in Benghazi, Libya, that took the lives of the US ambassador and three other Americans.

Republicans did not wait for Clinton's announcement to campaign against her. The party's chairman Reince Priebus said that voters could not trust Clinton and her election would be tantamount to giving Obama a "third term."

"Clinton's announcement comes in the shadows of looming investigations over deletion of State Department records and suspicious foreign donations," Priebus said. "For weeks Clinton has stonewalled the American public on unanswered questions around these many scandals. As an official candidate, Clinton must come clean with the American people."

Some Republicans sought to make foreign policy an issue at a time when the Obama administration is negotiating a nuclear deal with Iran and moving to restore diplomatic relations with Cuba.

"We must do better than the Obama-Clinton foreign policy that has damaged relationships with our allies and emboldened our enemies," said former Florida Gov. Jeb Bush, in his own online video Sunday. Bush, the brother and son of former presidents. is widely expected to join the race for the Republican nomination.

In the face of those negatives, Clinton has huge support among Democrats, and supporters began building a campaign base well before the official announcement.

Clinton's message will focus on strengthening economic security for the middle class and expanding opportunities for working families. The campaign is portraying her as a strong leader who can get results and work with Congress, business and world leaders.

Clinton's strategy, described ahead of the announcement by two senior advisers who requested anonymity to discuss her plans, has parallels to Obama's approach in 2012. He framed his re-election as a choice between Democrats focused on the middle class and Republicans who sought to protect the wealthy and return to policies that led the country into recession.

Still, Clinton faces pressure from the progressive wing of her party to adopt a more populist economic message focused on income inequality. Some liberals remain skeptical of Clinton's close ties to Wall Street donors and the centrist economic policies of her husband's administration. They have urged her to back tougher financial regulations and tax increases on the wealthy, while opposing new international trade agreements such as the Trans-Pacific Partnership.

"It would do her well electorally to be firmly on the side of average working people who are working harder than ever and still not getting ahead," said economist Robert Reich, a former labor secretary during the Clinton administration.

But Clinton appears unlikely to face a formidable primary opponent, though a handful of lower-profile Democrats such as former Maryland Gov. Martin O'Malley have said they are considering campaigns.

Should she win the nomination, Clinton would face the winner of a crowded Republican primary field that could feature as many as two dozen candidates.

Kentucky Sen. Rand Paul, a favorite among libertarians, and Texas Sen. Ted Cruz, a champion of the ultraconservative tea party movement, have already entered the Republican race. Cuban-American Sen. Marco Rubio of Florida is expected to announce his bid to be the first Hispanic president on Monday.