Updated: 5:40 p.m.

Campaign finance reports filed Wednesday yet again point to the basic problem facing Democrat-friendly super PACs as they struggle to catch up with their GOP counterparts: Wealthy liberals are still not playing the big-money game.

It’s not that high-rolling Democrats have stopped making campaign donations altogether. If anything, May fundraising totals just filed with the Federal Election Commission suggest that top-tier progressive donors are finally starting to open their checkbooks.

Priorities USA Action, the leading super PAC backing President Barack Obama, collected $4 million in May, not too much less than the $4.96 million raised by Restore Our Future, the super PAC supporting presumed GOP presidential nominee Mitt Romney.

The Priorities USA Action haul represents a big jump over the $1.6 million the group raised in April and looks even more impressive when compared with the measly $58,815 that the super PAC netted in January. June, moreover, is already turning out to be “a stronger month than May,” a Priorities spokeswoman said.

The problem for Democrats is that Priorities USA Action and other Democratic super PACs remain so dramatically outgunned by GOP-friendly outside groups. Restore Our Future, for example, has brought in $66.4 million so far in this election cycle, more than three times the $18.6 million Priorities USA Action has raised, according to the Center for Responsive Politics and public records.

The powerhouse GOP super PAC American Crossroads has pulled in another $34.5 million, and that super PAC’s affiliated nonprofit, Crossroads GPS, has raised at least $70 million more. (The nonprofit doesn’t publicly report its totals, but Crossroads operation organizers disclosed this spring that the two groups had brought in $100 million.)

The big reason Republican-friendly super PACs are so far ahead is the seven- and eight-figure contributions rolling in from billionaires and CEOs in the finance, real estate and oil and gas industries. A dozen donors have each given $1 million or more to Restore Our Future, according to the CRP, and more than one-third of the super PAC’s take comes from donors in the securities and investments industry.

Top GOP donors include Texas homebuilder Bob Perry, who has given $4.8 million to Restore Our Future and another $2.5 million to American Crossroads, the CRP data show. Casino mogul Sheldon Adelson and his wife will reportedly soon give another $10 million to the pro-Romney super PAC. Last month, Restore Our Future’s donations included $500,000 apiece from Rocco Ortenzio, executive chairman of Select Medical, a hospital holding company in Mechanicsburg, Pa., and Warren Stephens, president, chairman and CEO of Stephens Inc., a financial services firm in Little Rock, Ark.

By contrast, the biggest donation to Priorities USA Action is $2 million from DreamWorks Animation CEO Jeffrey Katzenberg. Additionally, the Democratic super PAC has received a handful of donations of $1 million or more, two of them from labor unions. The latest public disclosures show three new $1 million donors: Franklin Haney, CEO of the real estate and investment firm FLH Co.; Steve Mostyn, a lawyer with the Texas-based Mostyn Law Firm, and Barbara Stiefel, a Florida retiree and major Democratic donor.

But still largely missing are the liberal philanthropists and executives who showered $396 million in unrestricted contributions on 527s in the 2004 elections, according to the CRP data. These include investor George Soros, insurance executive Peter Lewis, film producer Stephen Bing and former banking industry CEO Herb Sandler and his now-late wife, Marion.

Together, those five donors gave $73.4 million to Democrat-friendly 527s in the 2004 elections, according to the nonpartisan Campaign Finance Institute. In this election,  Sandler have given no super PAC money, and Soros, Lewis and Bing have collectively donated $775,000 to such PACs — no pittance, but only a little over one-tenth of what they gave in 2004.

Democrats offer various reasons why large progressive donors haven’t ponied up this time. Many say donors felt burned after their 2004 investment in 527s — political organizations that may raise unrestricted money but must steer clear of explicit election messages and report their donors to the IRS. Their candidate, then-Democratic presidential nominee and Sen. John Kerry (Mass.), lost.

FEC fines imposed on several 527s drew negative publicity. Progressive donors remain philosophically opposed to unrestricted super PACs, despite President Barack Obama’s reluctant embrace of such groups in February. Most importantly, big liberal donors have strategically shifted their focus away from high-dollar PACs, which line the pockets of political consultants, in favor of investing in grass-roots groups organized around issues, many Democrats say.

Officials at Priorities USA Action acknowledge that GOP groups will outspend them this year, but they emphasize their recent successes. The group has aired $15 million worth of ads by leveraging partnerships with progressive allies, including labor and environmental groups, organizers say. The super PAC plans to spend another $10 million on the air between now and Labor Day, bringing its total broadcast spending to $25 million.

At this point in the 2010 elections, the Priorities spokeswoman noted, American Crossroads had raised only $1.3 million. Now the group has almost $30 million in cash on hand. It remains to be seen whether once-generous progressive donors will help Democratic super PACs catch up in time for Election Day.

Updated: 5:40 p.m. | This column has been updated to reflect that Marion Sandler passed away in early June at age 81.