Twitter today unveiled its “Political Index”, an attempt to compare sentiment about President Obama and Mitt Romney to the overall mood of the massive micro-blogging universe.

The Twitter Political Index — or “Twindex” for short — is born of a longstanding desire within the company to take a 50,000-foot view of its huge stores of data in hopes of gleaning conclusions about the public sentiment toward the presidential campaign, according to Adam Sharp, the head of government, news & social innovation at Twitter.

“There has been a dramatic expansion of the sample size for doing research on this data,” Sharp said in an interview with the Fix. “We now have enough data to start forming real-time data for the election.”

To illustrate the rapid expansion of Twitter, Sharp noted that on Election Day 2008 there were a total of 1.8 million tweets — on every subject — sent worldwide. That now represents six minutes of current Twitter volume. Six minutes!

Here’s how the Twindex works.

Twitter — using a data analysis company called Topsy — takes in the roughly 400 million(!) tweets sent each day. Using those tweets, they create a 1-100 sentiment index with “1” representing the most negative thing said on Twitter on any subject (that must be one hell of a tweet) and “100” representing the most positive thing said on Twitter.

So, Obama’s Twindex score of 34 means that 66 percent of all tweets on all subjects are more positive in tone than those about him while 33 percent are more negative. Get it?

“It’s not a head to head number,” explained Sharp. “It’s more typical of an individual approval rating.”

Sharp also acknowledged that the Twindex is far from a perfect measure of public sentiment about the candidates.

First, studies have shown that — at most — one in every six (or so) Americans are on Twitter, meaning that its far from a statistically sound sample of the American electorate.

Second, comparing sentiment regarding Obama/Romney to sentiment about Justin Bieber, Lady Gaga or the London Olympics may not really tell us much meaningful about how the public perceives both men.

“The Twitter Political Index is not meant to replace traditional’s meant to reinforce it [and provide] a more complete picture of the electorate,” said Sharp.

(Worth noting: Twitter is using Mark Mellman, a Democratic pollster, and Jon McHenry, a Republican pollster, to advise them on the Twitter Political Index.)

As evidence of the value of the Twindex, Sharp points to its correlation to Gallup tracking polling over the past few years.

Here’s a chart detailing that correlation since August 2010:

“There are days when we have it really right,” said Sharp. “There are months-long periods where our data tracks as closely as you can with Gallup.”

Given the exponential growth of Twitter in the past few years — and its primacy in the political-media world — attempts to measure what tweets tells us about ourselves and about where certain political storylines may be headed is of real value.

In short: Ignore the “Twindex” at your peril.