WASHINGTON – Former Massachusetts Gov. Mitt Romney, the closest to a front-runner in a wide-open Republican field, took a major step toward a second White House candidacy Monday, formally announcing a campaign exploratory committee.

Romney declared that “with able leadership, America’s best days are still ahead,” vigorously asserting that President Barack Obama had failed to provide it.

The Republican, who has been plotting a comeback since losing the GOP presidential nomination to John McCain three years ago, offered himself as the person best able to lead a country struggling to recover from economic crisis.

“It is time that we put America back on a course of greatness with a growing economy, good jobs and fiscal discipline in Washington,” Romney, a former venture capitalist with a record of turning around failing companies, said in a video posted on his website and on Facebook. He also announced the formation of the committee, which will allow him to raise money, in a Twitter message.

Romney’s move had been expected and a full-fledged campaign is a near certainty. He has traveled across the country to meet in private with donors and sound out their support. His political committee’s headquarters near Boston has been bulking up.

Romney’s strengths are substantial: He’s well known and he’s an experienced campaigner. He has a personal fortune and an existing network of donors.

But his challenges are big, too. They include a record of changing positions on social issues including abortion and gay rights, shifts that have left conservatives questioning his sincerity. He also has struggled to allay some skeptics of his Mormon faith.

Romney oversaw a health care law enacted in Massachusetts that’s similar to Obama’s national health overhaul, which conservatives despise. His announcement video didn’t mention either law.

He invested more than $40 million of his own money in the 2008 race and counted on early wins in Iowa and New Hampshire that never materialized.

Former Minnesota Gov. Tim Pawlenty has also taken the initial steps toward a White House run in 2012, setting up an exploratory committee. Former House Speaker Newt Gingrich is laying the groundwork for an early May announcement.

Mississippi Gov. Haley Barbour, former Sen. Rick Santorum of Pennsylvania and Minnesota Rep. Michele Bachmann are also putting together political machines for potential presidential runs.