We were recently made aware that @alhousedems had received a barrage of unexpected followers that look like bots. Scrolling through the recent followers, this does indeed appear to be the case. (Thanks @DePoInLa and @bweaver!) 
The bots in this set are easily identifiable - they all use the default color scheme but have changed their picture, usernames are lowercase and end in numbers with 3 or fewer digits, no bio, and no tweets. @TwitterSupport
The bots follow users other than @alhousedems, and checking the recent followers of the other accounts they follow reveals that @alhourdems is not the only target. We're still rounding out the set, but have found 29838 so far after looking at 20 accounts.
This chart shows the distribution of the creation times of the bots identified so far. The numbers have gotten bigger as we've expanded the dataset, but the curve has remained the same shape. We'll be updating this thread periodically as we continue to identify more bots.
Many thanks to @ZellaQuixote for proofreading this thread. You know who to blame (or credit) for any remaining typos.
Update - after checking the followers of 40 of the targeted accounts, we're up to 48254 bots and counting. This probably goes without saying, but if someone's creating over a thousand Twitter account per second, they're not doing it manually.
*accounts
So far, we've checked the follower lists of 60 targeted accounts and found 58886 bots. We're seeing more frequent repetition as we go (some of the bots are the same ones we've already found), but haven't yet reached the point of diminishing returns."
Can we estimate the overall size of this group of bots? We've found 62611 total bots following 70 target accounts, each followed by an average of 1836.31 bots. Those we've found thus far follow an average of 1836.31 * 70 / 62611, or 2.053 of the target accounts we've checked.
Overall, the bots follow an average of N target accounts. We've thus checked (approximately) only 2.053 / 169.5, or 1.21% of the total target accounts, and the estimated total bot population is 62611 * 169.5 / 2.053 or 5170104 total bots.
The number in the previous tweet seems quite frankly insane, but the math is correct is far as I can tell. If there's an error lurking somewhere, a correction would be appreciated.
Update: Twitter seems to be doing pretty well at cleaning these bots up, and not just the ones that were following @ALHouseDems. 50154 of the 62611 accounts we identified (80.1%) have been shut down thus far.
Another update - the bots are being replaced. @TwitterSupport
Footnote: the math in this tweet is based on an assumption of absolute randomness in terms of which targets each bot is assigned to. The data we already have suggests this isn't the case (we'd have found about twice as many bots after checking 70 targets).
The new bots following @ALHouseDems today appears to be a new batch created 4/26 (yesterday's were created 4/25). Checking 15 of their targets yields 23175 bots matching the pattern described in this thread, and there are quite a few more.
Another update - Twitter appears to have banned this wave too.
