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We know that the Santorum campaign is in a slow downward spiral. He's out of money. He's out of momentum. And he's almost  out of time.

But  the Romney camp has seen this movie before. And it knows that letting the Santorum campaign unravel at its own rate is a huge risk

Team Romney ignored Newt Gingrich, only to watch him win South Carolina. Millions of dollars later, the Romney camp- -and his friends at Restore Our Future - effectively took out Gingrich, but they forgot all about Santorum who went on to win Colorado and Minnesota. As we've noted many times before, Romney wins when he disqualifies his opposition in a barrage of negative attack ads.

Asked what he'd do if he were running the Romney campaign, one GOP strategist we spoke with told us, "I would crush him." Santorum has made Pennsylvania a must-win state, siad this strategist, "so don't let Rick win there."

So, while the Romney campaign has pulled this attack ad out of deference to the Santorum family, it's clear that it plans to  unload a new wave of attacks on the former GOP senator at some point before the April 24 primary.

Read more about Romney's decision to pull the ad here.

But  with his negatives sky-high and Republican voter enthusiasm dampened by a negative, drawn-out primary process, Romney needs to find a way to unite the party by giving GOP voters a reason to support him instead of just a reason to vote against someone else.

Another unaffiliated strategist we spoke with argued that Romney should "run a gazillion dollars in positive Romney ads." This strategy, argues this source, "is probably enough to win or at least get close, and it helps him in an important battleground."

How voters in the Philadelphia media market go in November, so goes Pennsylvania. Turning off those voters now may make it tough to get them back this fall.

Even so, if Romney can finally push Santorum out of this race post Pennsylvania, it means he'll have a lot more time to rehab his image this spring and summer.