Politicians are using sophisticated targeting tactics to send tailored online ads to different voters.

As we've been reporting, these ads are hard to track, and campaigns usually refuse to discuss their targeting tactics. This makes it hard to know who politicians are targeting online — and whether they're sending different messages to particular groups of voters.

We think it's important to keep an eye on what campaigns are doing, so we're collecting examples of targeted ads from across the country. We need your help.

To participate, there are just three quick steps:

1. If you spot a small blue triangle icon on any online political ad, or the words "Ad Choices," take a screenshot of the ad.

2. Click on the blue triangle or the words "Ad Choices" to find out which company showed you the ad. Take a screenshot of that, too.

3. Email the screenshots to us at targeting2012@propublica.org. Please include the full URL of the page where you saw the ad.

If the ad asks you to "learn more," visit a website, donate, or sign a petition, please send us a screenshot of that site or donation page, as well. (The page where the ad sends you may also be targeted in different ways to different visitors.)

This information will help us learn the most about how you were targeted. Here's what an ad and a disclosure may look like:

Not sure how to take a screenshot? Here are the instructions if you're using a PC, using a Mac, or using a smartphone.

We won't use any personal information without contacting you first, and we'll follow up with some questions about why the ad might have been targeted to you.

By collecting and sharing targeted ads, you can help us keep campaigns accountable and open a window into the rapidly evolving world of online advertising.

We've already discovered that Mitt Romney ads may follow some users across the Internet, as well as the fact that "dark money" nonprofits like Crossroads GPS are now using data about Internet users to send online ads to certain people. We've explored how Microsoft is using the names and Zip codes it gets from Hotmail users to help campaigns — including the Obama campaign — target their ads more precisely.

But there's much more to find out — including exactly what kinds of targeted ads the Obama campaign is using, and which local politicians are adopting these targeting tactics.

Get a lot of email from political campaigns? You can also check out our "Message Machine" project, which analyzes how campaigns are targeting voters with different email messages.

To see all of our coverage on the campaign, including our Follow the Money interactive, visit our Campaign 2012 page.

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