Romney's campaign was the first presidential campaign to use the promoted trend tool. According to Ad Age, promoted trends sell for between $100,000 and $120,000, so only national campaigns are likely to use them. Twitter began offering the tool last year.

The hashtag #Forward2012, Obama's campaign slogan, appears now at the top of Twitter's list of worldwide trending topics, along with a purple "promoted" symbol that indicates the hashtag was paid for by Obama for America.

Obama's campaign is also promoting a tweet within that trend that links to a comparison between Obama's "vision for America" and its description of Romney's. Obama's proposals are listed under "Forward" and Romney's under "Not backward."

Obama's campaign has already seen success on Twitter when it comes to people talking about the convention this week. Both nights of the Democratic convention have prompted more tweets than the peak night of the GOP convention last week. First lady Michelle ObamaMichelle LeVaughn Robinson ObamaMichelle Obama says upcoming memoir shares the 'ordinariness of a very extraordinary story' Colbert: Melania Trump’s jacket was ‘one message she did not steal from Michelle Obama’ Melania Trump puzzles with 'I really don't care' jacket MORE's speech to the convention on Tuesday, the opening night, prompted nearly twice as many tweets per minute as Romney's acceptance speech last Thursday. More people tweeted about her than about former President Clinton, who headlined the second night but was competing with the first game of the NFL season.

By the end of night two of the Democratic convention, more than 5.5 million tweets had been sent about it, according to Twitter, in contrast with about 4 million total for the GOP gathering last week.

That could be partly because when it comes to using Twitter, Democrats tend to see it as a more important tool than do Republicans, according to a Pew Internet and American Life Project study released earlier this week.