One of the most potent outside liberal-oriented political groups announced on Thursday that it would back Hillary Rodham Clinton if she chooses to seek the White House in 2016, the latest sign that senior members of President Barack Obama's team were looking to line up with the former first lady.

Priorities USA brought on board Jim Messina, Obama's 2012 campaign manager and the latest Obama insider to join a pro-Clinton group. With so many Obama veterans lining up to help clear the way for a Clinton campaign, it perhaps adds pressure on Clinton to go forward with another White House bid.

Clinton, a former secretary of state, senator and first lady, has not yet decided if she will again seek the White House. But that has not stopped supporters from building a political machine in case she decides to turn the key.

The early alignment for Priorities USA sends a clear signal that high-dollar donors were awaiting a Clinton decision and were ready to fork over millions to promote and defend Clinton from Republican criticism that conservative groups have been leveling for months, trying to weaken her standing should she choose to run.

Other pro-Clinton outside groups have been forming and working to defend her record in her absence. Correct the Record, for instance, has been trying to debunk Republicans' criticism of her decades in public service while Ready for Hillary has been organizing low-dollar fundraisers for her aimed as much at showing grassroots support as building a list of would-be-volunteers if Clinton joins the race.

But the formal shift for Priorities signals that big-dollar donors were ready to join the Clinton bandwagon.

Priorities helped raise millions for Obama's re-election and ran some of the election's toughest television ads defining Republican nominee Mitt Romney as a heartless corporate raider.

After 2012, the group considered various options but its donors clamored for Priorities to become the de facto pro-Clinton super PAC.

The addition of Messina and former Michigan Gov. Jennifer Granholm, which had been expected, puts the leadership of the super PAC in the hands of one of Obama's most trusted advisers and a longtime Clinton supporter.

Before directing Obama's re-election campaign, Messina was a top 2008 campaign aide and a White House adviser who played a key role in the passage of the president's health care overhaul. Granholm had previously announced her support of Clinton through a separate super PAC, called Ready for Hillary, and had supported Mrs. Clinton during the 2008 Democratic primary.

Messina joins a number of top former Obama campaign aides who have backed outside efforts to bolster Clinton's standing should she run for president. Senior campaign aides Jeremy Bird and Mitch Stewart are advising Ready for Hillary. Buffy Wicks, who led the campaign's voter mobilization effort, recently signed on as executive director of Priorities USA Action.

The early efforts have begun to create an impression that Clinton is the favored heir to Obama's White House, as opposed to Vice President Joe Biden, who has kept open the option of a third White House campaign.

The outside groups have formed an early de facto campaign organization to help Clinton compete vigorously against Republican opponents who are already trying to negatively define her tenure as secretary of state. The organizations are building the network without her direct consent but she has not raised any objections, offering a tacit endorsement that has encouraged Democrats to push forward in preparation for a campaign.

The group's alignment had been expected for months but first confirmed Thursday by the New York Times. Peter Kauffmann, the group's spokesman, confirmed the group would be supporting Clinton if she runs.