Sixty percent of Milwaukee's black voters have disappeared.

Democrats have feared for years that one of the particular challenges of running campaigns in 2012 would be simply locating their voters. The party's constituencies (young people, immigrants, minorities) tend to be among the most mobile demographic groups.

And as NPR speculated this week in an analysis of battleground-state foreclosure figures, the housing crisis will likely only have made things more difficult for Democrats looking for their supporters.

New data from Milwaukee give an indication of how dire the Democrats' disappearing-voter problem already is. This spring, the League of Young Voters, which was created to mobilize young minority communities, collaborated with the liberal Wisconsin Voices coalition to dispatch teams of young canvassers.

Starting in April, they spent eight weeks knocking on 120,882 doors across 208 of Milwaukee's 317 wards to raise awareness of the gubernatorial recall election scheduled for June. The doors had one thing in common: the voter file said they were all home to a registered voter whom a commercial data vendor had flagged as likely to be African-American.

But the voter file represented a fiction, or at least a reality that had rapidly become out of date. During those eight weeks, canvassers were able to successfully find and interact with only 31 percent of their targets. Twice that number were confirmed to no longer live at the address on file - either because a structure was abandoned or condemned, or if a current resident reported that the targeted voter no longer lived there.

Based on those results, the New Organizing Institute, a Washington-based best-practices lab for lefty field operations, extrapolated that nearly 160,000 African-American voters in Milwaukee were no longer reachable at their last documented address - representing 41 percent of the city's 2008 electorate.

It is a staggering figure in a battleground state where Democratic prospects rely on turning out Milwaukee's urban population, an ever more urgent cause since Paul Ryan's presence on the ticket could help mobilize core Republican constituencies in the city's suburbs. Over half of those identified as displaced were under the age of 35, and thus also less likely to be reachable through traditional landline phones.

The Milwaukee data will certainly be sobering to Democrats who rely on existing voter-file records when organizing walk sheets and call lists for get-out-the-vote canvasses. Now, those working in Wisconsin realize, they'll have to begin the process earlier to pinpoint residents and make sure that those who have been relocated are registered to vote at new addresses.

"That scraps any traditional GOTV planning - if six in 10 people that you planned on talking to are not there," says Biko Baker, executive director of the League of Young Voters.