In partnership with social analytics service Topsy, Twitter launched a Political Index that measures sentiment towards Barack Obama and Mitt Romney.
Each day, the Index evaluates and weighs the sentiment of Tweets mentioning Obama or Romney relative to the more than 400 million Tweets sent on all other topics. For example, a score of 73 for a candidate indicates that Tweets containing their name or account name are on average more positive than 73 percent of all Tweets.
The key is the comparison against all tweets for a sense of scale. As seen from the chart below, the index fluctuates closely with Gallup estimates.