Priorities USA Action, the leading Democratic-affiliated super PAC, collected $4 million in May, its best month of fundraising since its inception and a sign, its founders argue, of its growing momentum.

The $4 million haul is roughly equivalent to what the group, which is run by two former Obama White House aides, collected in April and March combined. Sources familiar with its fundraising operation insist that June fundraising will eclipse May. The group has raised $40 million total to date.

The fundraising from Priorities has to be encouraging for Democrats who openly fretted about their ability to keep up with the well-funded conservative super PACs led by American Crossroads and Restore Our Future, which announced today it had raised $5 million in May.

American Crossroads raised $1.2 million in March and $1.8 million in April. The group has yet to release its May fundraising report.

What’s nearly certain to be the case, however, is that Crossroads’ cash on hand total will dwarf that of Priorities, which ended May with $4.5 million on hand. At the end of April, Crossroads had $25.5 million in the bank.

Priorities USA has spent $15 million to date on a series of television ad campaign in conjunction with other members of the progressive coalition. The largest chunk of that money — roughly $10 million — has been spent on an in-progress ad buy in Colorado, Florida, Ohio, Pennsylvania and Virginia that attacks former Massachusetts governor Mitt Romney on his time spent at the head of Bain Capital.

Priorities sources note that the bulk of the anti-Bain messaging against Romney has been carried by their group with the Obama campaign spending roughly $100,000 on its own ads on the topic.

With the Obama campaign publicly acknowledging earlier today that they will be outspent on television in the general election, the success of Priorities USA Action is absolutely essential to fighting Crossroads if not to a draw than something close to it.

May was Priorities best month yet. But they start in a deep financial hole against Crossroads, which has spent more than two years honing its fundraising and political apparatus. Whether they close the gap enough to make a difference between now and November still remains to be seen.