Political candidates (and their pollsters) have a new popularity contest to worry about.

Twitter's government and politics team have announced the launch of the Twitter Political Index--a new barometer of what the public is thinking (and, of course tweeting) about President Obama and Gov. Mitt Romney.

The index, built in partnership with social web search company Topsy and polling firms The Mellman Group and North Star Opinion Research, will evaluate the tone of tweets mentioning Obama or Romney. The resulting index scores indicate how positive how positive their Twitter mentions are relative to all tweets.

A candidate's score of 50, for example, would mean that tweets including his name or Twitter handle were more positive than 49% of all tweets sent on a given day.

Twitter says the index aims to provide more nuance to political coverage. illustrating instances in which the social media conversation diverges from more traditional polls.

For instance, Twitter notes, retroactively calculated index scores for President Obama over the past two years would have been basically in line with Obama's Gallup approval rating--but the two gauges went in opposite directions in September 2011, suggesting that in the online community, public opinion of Obama was different from what Gallup was reporting.

The index will be update at 8 p.m. EDT each day, Twitter says. As of Thursday afternoon, Obama had a pronounced lead over Romney, with index scores of 44 to 26.