Why is Rick Santorum surging in national polls and leading in key primary states? A big part of the explanation is that, during the long months when he campaigned relentlessly despite low poll numbers and a lack of major media attention, Santorum’s operation developed a resourcefulness that his rivals don’t have.

Necessity is the mother of invention, and the low-budget Santorum campaign — which kept going throughout 2011 on a mere $2.2 million — was required to find ways to build a successful grassroots organization without the big money other Republican candidates were collecting.

Now that he is starting to look like a winner, serious money is coming in. My estimate is that the Santorum campaign got more than $5 million in the week following his Feb. 7 “trifecta” wins in Missouri, Minnesota and Colorado. The thrifty habits of the Santorum operation enable them to extract maximum value from their contributions, and an ABC News report about the GOP campaign finance report for January highlights Santorum’s advantage:

Rick Santorum for President
Raised: $4.51 million
Spent: $3.32 million
Cash on hand Jan. 1: $279k
Cash on hand Jan. 31: $1.47 million
Debts/obligations: $0

Romney for President Inc.
Raised: $6.54 million
Spent: $18.78 million
Cash on hand Jan. 1: $19.92 million
Cash on hand Jan. 31: $7.68 million
Debts/obligations: $0

Newt 2012
Raised: $5.59 million
Spent: $5.91 million
Cash on hand Jan. 1: $2.11 million
Cash on hand Jan. 31: $1.79 million
Debts/obligations: $1.73 million

In January, Gingrich spent everything he raised, and more, and finished the month with debts nearly equal to his cash-on-hand figure. Newt’s campaign was essentially broke after the Florida primary Jan. 31.

Meanwhile, Romney’s campaign had a negative “burn rate” — they spent more than they took in — to the tune of $12 million. And as Allahpundit noted in an item I linked in the headlines, for Romney to report a mere $7.7 cash-on-hand total as of Jan. 31 is very bad news for Team Mitt.

Ask your seventh-grader to do this math problem: If your spending exceeds your income by a margin of $12 million a month, how many days will $7.7 million last?

If the only way Romney wins primaries is when he can bury his opponents in an overwhelming avalanche of attack ads, his ability to win primaries may be nearly exhausted.

Of course, the pro-Romney “super PAC” entered February with a fat $16.3 million in the bank, but even his super PAC had a negative “burn rate,” spending more than twice as much in January ($13.9 million) as they collected in contributions ($6.6 million).

What does this mean? Right now, the Romney campaign is trying to hold on in his home state of Michigan with another one of its customary attack-ad blitzes against Rick Santorum, but Team Mitt can’t spend the kind of money now that they spent against Gingrich in December and January. Yet even if Romney can manage to win Michigan, his campaign may be headed toward a serious financial crunch before the “Super Tuesday” primaries in March.

Thus, when you look at the campaign cash numbers, the chatter about Romney’s “electability” is much less plausible, Sheldon Adelson’s big bets on Gingrich look like wasted money, and the fact that Santorum is gaining new “super-PAC” donors may be far more significant than it appears. Despite everything, Romney may yet prevail, but as we enter February’s final week — on the eve of Wednesday’s debate in Arizona — he certainly is no longer “inevitable.”