Today’s New York Times reports an eye-popping figure: The pro-Romney forces may be able to raise and spend no less than $1 billion to oust President Obama. That number could climb considerably higher, if the fundraising by Romney-aligned outside groups is a success.

Which raises a question: Why is Dem outside-group fundraising failing to keep pace?

“The fact that the Romney forces are going to be able to amass at least $1 billion dollars should be a tremendous wake up call for our side,” Paul Begala, an adviser to Priorities USA, the main pro-Obama outside group, tells me.

Media reports have speculated that the Obama campaign will raise $1 billion for reelection, but the campaign vigorously disputes this. Meanwhile, Priorities USA and Priorities USA Action have only raised a combined $10 million. That sum was matched by a single anonymous GOP donor who quietly handed the Rove-founded Crossroads GPS a $10 million check the other day.

I asked Begala why Dems are lagging behind. He gave three reasons. The first: Dem donors are used to an outdated calendar. The air wars designed to establish the frame of the election are getting under way far earlier in the new era.

“Dems are used to giving late — we’d have an ad blitz in the last eight weeks, but before then it would be all about building up the ground game,” Begala said. “Now we’ve got a different timetable. We want to define Romney now, as he’s trying to define himself.”

“The second reason is that our donors are not transactional,” Begala continued. “For the Koch brothers, this is a straight return on an investment deal.” A Republican president, Begala continued, “would preserve tax breaks for their oil companies and kill off Obama’s investments in green energy.”

Begala’s third reason: “Many progressive donors think President Obama has it in the bag. But he doesn’t even have it in the shopping cart yet, much less in the bag.”

There are mitigating factors. Major labor unions will also be raising and spending huge sums. But some union resources may be diverted elsewhere. Other Dem groups are raising cash for the Congressional races. But the pro-Romney forces may have well over $1 billion for the presidential race alone.

True, there’s been some debate over how much outside spending really matters. But this is the first presidential race in which we’re going to see this level of spending, so who knows how it will turn out. Dem donors may not be aware of the spending juggernaut that’s about to hit them.