By Brendan O'Brien
MILWAUKEE (Reuters) - Wisconsin Democrats won a preliminary victory on Tuesday in their bid to unseat Republican lawmakers who voted for a controversial anti-union law in the first of a series of special recall elections.
Party-backed candidates won in each of six state Senate districts in Tuesday's vote, essentially a Democratic primary but with an odd twist, that took place ahead of a formal August recall vote.
The candidates had to beat back unusual primary challenges from six Republicans who ran as Democrats in order to give the targeted Republican incumbents more time to campaign, raise money and maintain their party's hold on the state Senate.
Unofficial results posted on websites run by election officials in the counties where the six races were run showed the Democratic Party-backed candidates won all six races.
Unlike many other U.S. states, Wisconsin has open primaries and no official party registration. So Republicans can run as Democrats and vote in Democratic primaries, and vice versa for Democrats.
The six Democrats who won on Tuesday -- Nancy Nusbaum, state Representative Sandy Pasch, Shelly Moore, state Representative Fred Clark, Jessica King and Jennifer Shilling -- will now seek to unseat six incumbent Republicans in recall elections on August 9.
In total, six Republican senators who supported the anti-union measure, and three Democrats who opposed it, will be forced to defend their seats this summer after recall petitions were signed by thousands of disgruntled voters.
(Writing by James B. Kelleher; Additional reporting by Karin Matz in Chicago, David Bailey in Minneapolis and Jeff Mayers in Madison; editing by Cynthia Johnston)