In a tightly-guarded surprise campaign stop, Mitt Romney visited failed energy company Solyndra Thursday and invoked the building as a symbol of what he called President Obama's misuse of taxpayer dollars.

The visit - kept a secret by the campaign until shortly before the candidate and the press arrived at the site - offered a hulking visual to accompany Romney's repeated criticism of Obama over the Solyndra scandal.

Standing on a gritty embankment across the street from Solyndra's edifice, Romney called the glass-fronted building the "Taj Mahal" of corporate headquarters and a symbol "gross waste" and accused the president of cronyism.

"This building, this half a billion-dollar taxpayer investment, represents a serious conflict of interest on the part of the president and his team," Romney said. "It's also a symbol of how the president thinks about free enterprise. Free enterprise to the president means taking money from the taxpayers and giving it freely to his friends."

The solar energy company based outside San Francisco went bankrupt less than two years after receiving a $535 million loan in 2009 from the Department of Energy.

The Thursday visit was kept a tight secret by the Romney campaign, which told reporters they needed to travel in a campaign bus to an undisclosed location.

A Romney adviser said the campaign had concealed the event location for fear the Obama administration would somehow prevent them from staging it.

During his news conference, Romney alluded to protests organized by Democratic activists that have disrupted several previous campaign events, and said his campaign had wanted to head off any efforts by his rivals to keep him from speaking.

"I think that there are people who don't want to see this event occur, don't want to have questions asked about this particular investment, don't want to have people delve into the idea that the president took a half a billion dollars of taxpayer money and devoted it to an enterprise that was owned in large measure by his campaign contributors," he said. "I think there are a number of people among the president's team that don't want that story to get out. We wanted to make sure it did."

Obama administration officials have called the failed Solyndra loan a disappointing result in their ongoing support of innovative technology and have vowed to continue investing in clean energy.

"Over the course of this first term, we will have doubled the use of renewable energy in this country," David Axelrod, Obama campaign senior adviser, said Thursday in an interview to air on CNN's "John King, USA."

He added Romney was simply using the Solyndra stop as a "diversion" from recent attacks against his corporate career in private equity

"One thing the president never did was buy a company, load it up with debt, bankrupt that company, and walk away with millions of dollars while the creditors and the workers were left holding an empty bag," Axelrod said, taking a shot at Romney's former firm, Bain Capital. "The president's never done that."

Romney's visit to Solyndra was the latest step in a continued push from Team Romney and Republicans to paint the president as harmful for job creation, which has used Solyndra as its central argument. The Republican National Committee released a video Thursday, which followed similar efforts from the Romney campaign and Crossroads GPS, an independent conservative group.