That endorsement happened late on Friday, and became a trending topic on Twitter.

But the bump for Romney didn't last. According to Twitter, he hit 39 on the Political Index Saturday but fell to 27 on Sunday. This means that tweets containing Romney's name or account name were rated more positive, on average, than 27 percent of all other tweets sent on Sunday.

Obama remained steady at 74 (out of 100) all weekend, and has led in positive buzz online since Twitter announced its daily tracking index last Wednesday.