While the presidential race has been pretty static lately and Republicans picked up a nice win in the Scott Walker recall — in which, as you know, Democrats were massively outspent — Senate campaigns have been trending towards the Democrats over the last few months. Now, the other shoe begins to drop: Karl Rove’s Crossroads is dumping $866,000 into contested Senate races in Indiana, New Mexico, and Montana.

The good news for Democrats is that Indiana is even in play. Remember, if Richard Lugar had been re-nominated, everyone agreed that the race would be over. Instead, Democratic Member of the House Joe Donnally is a viable enough threat that this outside group believes it’s necessary to launch an attack. The other two races have Republicans looking for gains: Senator Jon Tester is one of the most vulnerable Democratic incumbents, and Republicans have a strong candidate in Heather Wilson as they try to pick up an open seat in Democratic-leaning New Mexico.

This all raises two questions. The first one is about the debate Greg’s been covering about how Democratic-aligned groups should be spending their money during this cycle: on “ground game” organizing or on advertising. One thing’s for sure; Republican-aligned ads are certainly going to be more visible than ever.

The second question is simply whether Democrats will be able to compete at all. That’s not an issue in the presidential race; Barack Obama’s campaign is going to be very well-funded, and at any rate money is clearly less important in presidential general election campaigns than anywhere else in the electoral system.

But at the Senate level, and even more at the House level, money can matter a lot — especially unopposed money. That’s why DCCC Chair Steve Israel is sounding the alarm right now by calling on Dem donors to get serious about funding the air wars, and not just on-the-ground organizing. I suspect that the overwhelming financial advantage for Scott Walker’s campaign was only somewhat effective in that contest, given the polarization of the state and the fairly heavy media coverage it received. But what we may see this fall are low-profile House and Senate races in which outside groups toss in enormous amounts of money that go virtually unopposed. That’s a recipe for money being decisive.

That will be the case unless Democratic-aligned groups are able and ready to step up to counter it. Whether that will happen is probably the biggest and most important unknown of the 2012 cycle — and could help determine control of the House and Senate.