White House attacking Trump to give his candidacy credibility?

I simply don't believe this claim:

"There is no issue that irritates the White House more than the birther issue," said Larry Sabato, director of the University of Virginia's Center for Politics.
"They view it as a symbol for those who refuse to acknowledge Obama as a legitimate president," he said. "The fact that someone as prominent as Trump is bringing this up is a source of real frustration."

I think that the Obama administration is attacking Trump precisely to give his candidacy credibility:

The White House stopped just short of dismissing Donald Trump as a clown Sunday - calling him a "sideshow" act with "zero chance" of becoming president.
Chief Obama adviser David Plouffe unleashed a barrage of stinging comments on Trump, who has recently trafficked in fringe conspiracy theories about Obama's place of birth while taunting America with hints of a presidential run.
"There is zero chance that Donald Trump would ever be hired by the American people," President Obama's chief adviser David Plouffe told ABC's "This Week with Christiane Amanpour."
Plouffe noted Trump's surprise second place showing in a recent poll of New Hampshire voters with glee.
"I saw Donald Trump kind of rising in the polls and given his behavior and spectacle the last couple of weeks, I hope he keeps rising," Plouffe said. . . .

Note the past Obama practice of praising those Republican presidential candidates whom they want to hurt:

President Barack Obama has come to praise his Republican challengers - and to bury them.
Over the last few weeks, Obama and his top allies couldn’t seem to stop applauding several of the GOP’s potential 2012 contenders.
To listen to them tell it, former Massachusetts Gov. Mitt Romney is a health care visionary and U.S. Ambassador to China Jon Huntsman is a loyal foot soldier of the administration.
The political calculus behind that praise is straightforward: by wrapping their arms around some of the GOP’s most credible and deep-pocketed potential challengers, Democrats undermine the party’s attempt to win over its conservative base in the primary.
Obama aides deny that they’re wading into the Republican contest, but they’ve sought to intervene in GOP politics in the past, if on a smaller scale. Part of the benefit of sending Huntsman to Beijing was the hope that it would remove a moderate and wealthy Republican from the 2012 field, just as appointing Rep. John McHugh (R-N.Y) as Secretary of the Army and attempting to place Rep. Peter King (R-N.Y.) and former Sen. Judd Gregg (R-N.H.) in the administration would have freed up GOP-held seats. . . .