For all the candidates in the presidential race a month ago, political advertising was running well below expectations.

That's about to change, and in the biggest way, as Democratic rivals Hillary Clinton and Barack Obama battle for their party's nomination.

Spending will be particularly intense in the coming month and possibly into late April, when Pennsylvania holds its primary.

Both candidates are amassing huge war chests, and most of it will go to TV in markets in big-delegate states like Ohio and Texas, that are critical for both.

The closeness of the race this late in an election cycle is unprecedented in recent history, and there's some concern the two could remain in a dead heat right into the convention, held in August, and spending heavily on advertising as they duke it out.

Just a few months ago, the presidential campaigns were projected to spend between $650 million and $800 million. Spending is now expected to be on the high end of that forecast, possibly going above $800 million. Total political spending, including local and regional races, as well as issue spending, will likely surpass $2.5 billion.

Several states where Obama is favored are holding contests in the next two weeks. But the biggest upcoming battles will take place next month in Ohio and Texas, which hold primaries on March 4 and where Clinton has the best shot of widening her tiny lead.

Each candidate had raised well over $100 million prior to Tuesday, but they were spending it as quickly as it came in, and at one point Clinton reached into to her own purse to keep her campaign going.

But then the day after Super Tuesday both saw great streams of fresh donations, $4 million for Clinton and $7 million for Obama.

“At this point, this has to be viewed as a net positive because you’ll have two candidates fully funded and fully spending,” says Evan Tracey, COO of TNS Media Intelligence’s Campaign Media Analysis Group.

Significantly, as the pundits point out, for all the bickering between the two candidates, Democratic voters remain positive about both. Rather than voting for one and against the other on Tuesday, they saw themselves as choosing the one they preferred.

That's a positive for both candidates and Democrats generally. It reduces the risk of the two turning off voters in the event they remain in a dead heat by the time of the convention.  

Going forward, Tracey notes, "The spending now will be very targeted, focused on battleground states.”

Both campaigns will heavily advertise throughout Ohio and Texas, including significant spending on Spanish-language media. Clinton has so far had a lock on Latino voters. Spending will then taper off after Pennsylvania.

As for the Republicans, their spending sprees are over, at least until the early fall, heading into the general election, now that John McCain is the clear frontrunner, with Mitt Romney out of the race and Mike Huckabee hanging in but without heavy backing to finance TV advertising.

In place of Republican spending, the so-called 527s--interests groups promoting various causes--will step in, spending through the summer in support of the Republicans while McCain and the party leaders strategize over November.

Democratic 527s would be doing the same if there were a clear frontrunner.

In any case, it promises to get nasty.

“What you’d be going into is the special-interest character assassination campaigns,” says Tobe Berkovitz, interim dean of Boston University’s college of communication. “Most of the presidential campaigns wouldn’t be on the air unless they were flush with cash.”

The big puzzle now is whether Clinton and Obama will vie all the way into the Democratic convention in August. If they were to do so, it would hurt the eventual nominee, leaving him or her only two months to prepare a campaign and advertise for the November election.

The talk now is of a deal, worked out by Democratic National Committee chairman Howard Dean, with one candidate agreeing to accept the second spot on the ticket as the candidate for the vice presidency.

But that would seem quite unlikely if the two candidates are still so close in their delegate counts.