ABC's News' Michael Falcone reports:

Rick Santorum's number crunchers are under no illusions that they are facing a tooth-and-nail fight with Mitt Romney over delegates, but their calculations show a significantly narrower gap between the two contenders than most estimates.

The Santorum campaign offered ABC News a sneak peek at their in-house delegate tally, which still shows the former Pennsylvania senator trailing Romney but in a much better position to catch him.

"There is the Romney way of going about the counting and then there is the real way of going about the counting," John Yob, Santorum's delegate strategist, said in an interview on Monday.

Here's how the Santorum campaign sees the standings in the race for delegates:

Romney: 435

Santorum: 311

Gingrich: 158

Paul: 91

The Santorum campaign's version of the count puts them 124 delegates shy of Romney. By comparison, the ABC News delegate estimate shows Santorum 268 delegates behind Romney.

Here is the ABC News delegate estimate, which tracks closely with tallied kept by other news organizations, including the Associated Press:

Romney: 521

Santorum: 253

Gingrich: 136

Paul: 50

Uncommitted: 2

A candidate needs 1,144 delegates to clinch the Republican presidential nomination, and Yob conceded that it is going to be "very difficult for any candidate" to reach that threshold before this summer's national party convention.

According to the Santorum team's count, Romney has 86 fewer delegates than in ABC's estimate; Santorum has 58 more; Newt Gingrich has 22 more; and Ron Paul has 41 more.

So, where does Yob, who played a similar delegate strategy role for John McCain in 2008, come up with the 144 delegate difference?

Their delegate equation largely rests on two key assumptions: First, that Arizona and Florida will eventually allocate their delegates proportionally rather than using their current winner-take-all scheme. Second, that delegate tallies in Iowa, Missouri and Washington State should be estimated based upon the preliminary results of ongoing county and district conventions, not on the initial "beauty contest" votes.

The Santorum campaign believes they will receive the vast majority of the delegates in Iowa and Missouri and they are seeing signs of encouragement in Washington State. In King County, which held legislative district caucuses this weekend to choose delegates to send to the state convention later this spring, Santorum netted four delegates, compared to three a piece for Romney and Gingrich and two for Paul.

(In Washington State's March 3 caucuses, Romney won King County by roughly 29 percent over Santorum.)

"We are now far exceeding the perceived delegate counts as laid out by the Romney campaign," Yob said. "This is just the beginning."

The campaign claims the upper-hand in the coming state conventions, which will award some bound and unbound delegates over the next few months. Santorum's team reckons more grassroots conservatives will show up - a group that has been friendlier to Santorum.

Yob's theory about Florida and Arizona is based upon the Republican National Committee's original rules that forbid states from holding winner-take-all contest before April 1. Both states leapt ahead on the primary calendar and chose to use a winner-take-all delegate rule. The Santorum campaign plans to challenge the allocation through a formal contest process before the convention.

"We believe that the RNC is likely to follow their own rules," Yob said. "And we believe that any effort by the Romney campaign to manipulate the rules will have a negative impact on their effort to get to a majority on the convention floor."

Current delegates tallies, including the RNC's show Romney receiving all 50 delegates in Florida and all 29 in Arizona. If the Santorum campaign prevails, they see a proportional allocation netting them seven delegates in Florida and 8 in Arizona as follows:

Florida:

Romney: 23

Gingrich: 16

Santorum: 7

Paul: 4

Arizona:

Romney: 14

Santorum: 8

Gingrich: 5

Paul: 2

Over the last week, Santorum aides have also been pushing the notion that by the end of the primary season they will have forged a "conservative majority" of delegates. That is, they view Gingrich as a marginal candidate whose supporters are more likely to flow to Santorum - an assumption that is not necessarily borne out in at least one recent national Gallup poll.

The campaign's model requires something of a leap of faith. Essentially, everything that could go right for Santorum must. Romney, meanwhile, continues to hold on to the undisputed delegate lead - a fact that the Santorum campaign does not deny.

Santorum advisers acknowledge that April could be a tough month with a slew of contests in the North East that seem ripe for Romney. However, they view May's contests, which include several southern states and Texas, far more favorably.

"The closer we get to a conservative majority," Yob said, "the closer we get to the nomination."

The Romney campaign disagrees.

"We knew Sen. Santorum was an economic lightweight, but his problems with numbers are worse than we thought," Romney spokeswoman Andrea Saul told ABC News. "At the end of the day, Gov. Romney's message of restoring America's promise will continue to resonate with more Americans than any other candidate."

ABC News' Elizabeth Hartfield and Chris Good contributed reporting.