The new rules will restrict ads for contentious subjects like abortion and climate change. Some keywords, such as "conservative" and "liberal," won't be allowed as a matter of course. The list of barred keywords will receive continuous updates, Twitter said. The company's new policies also curb the use of microtargeting, or attempts to skew elections by aiming ads at narrow demographics. Issue advertisers won't be allowed to target ads based on criteria like age, ethnicity and specific location, although state-level targeting will be permitted.

Like politicians, issue advertisers won't be allowed to champion specific political actions. News publishers who are already exempt from Twitter's issue rules (at least 200,000 visitors in the US, not primarily user-submitted and not focused on one issue) are allowed to advertise based on their fact-based reporting, but not to endorse candidates or banned topics.

There are concerns about how well Twitter will enforce the new policies. Facebook blocked innocuous LGBT ads due to its approach to issue ads -- will Twitter risk similar problems? There are also questions as to whether Twitter will have a consistent definition of fact-based reporting in light of allegations of political bias. Legal and policy VP Vijaya Gadde said Twitter was prepared for the possibility of "[making] some mistakes," though, and made clear that the site would have to "improve this policy over time." If you're not a fan of the rules as they are, don't be surprised if they evolve before long.