Twitter is selling political advertising beginning this week.

Politico reports that the microblogging service has signed up five presidential campaigns for the launch. Sponsored political tweets will be distinguished with a purple check mark.

As with Google ads, rates are determined by advertisers bidding on search terms.

Twitter is not, however, offering campaign ads inserted directly in users “timelines” – the streams of short updates from the group of people each user chooses to follow, though the company has begun cautiously to experiment with that model for commercial advertising.

They’re instead offering three less obtrusive products: Promoted tweets, which look like non-commercial tweets except for a small logo and disclaimer, and which appear when a certain term is searched for and will appear in a campaign’s follower’s timelines no matter when the log in; promoted trends, which allow an advertiser to appear atop popular lists of “trending topics”; and promoted accounts, in which the campaign’s account is suggested to Twitter users who appear to have similar interests.