Countdown begins for drive to recall Wisconsin Gov. Scott Walker

An effort to recall Scott Walker, the Republican governor of Wisconsin, began at midnight Tuesday morning, starting the 60-day clock by which opponents must gather signatures necessary to prompt an election to remove Walker from office.

Approximately 540,000 valid signatures must be collected by Jan. 14 to permit a vote on recalling the governor-- who successfully pushed to end collective bargaining rights for most state workers during a showdown earlier this year that gained national attention and made Walker into a target for Democrats and labor activists around the country. The 540,000 number is one-quarter of the votes cast in the November 2010 election in which Walker won his first election as governor.

In a sign of Walker's concern over the recall, the governor's campaign began airing ads Monday night that positively promote Walker.

"I'm committed to working together to create more jobs, to improve our schools and protect our seniors," Walker says in the first ad, which features a mother who is a school board member touting the effect Walker's budget reforms have had on schools.

Watch the ad below:

During Monday night's Green Bay Packers game, a plane carrying a banner that read "KICK OFF THE RECALL" flew overhead and was captured by news photographers.

Many groups have mobilized for the recall effort, including the leading organization, "United Wisconsin," which comprises more than 200,000 residents and organizations. The group organized midnight petition rallies, which were promoted as a chance to be among the first to support the recall of Walker and Lt. Gov. Rebecca Kleefisch.

United Wisconsin claims that on Monday afternoon their website--which offers downloadable petitions and other recall materials--was hit with a "cyber attack."

"Our web team reports that this attack was coordinated and deliberate [in] its target," the group announced on its Facebook page. "Clearly this was done to deny the people of Wisconsin access to the materials we are providing for the Walker recall efforts. The attack, what is known as a Distributed Denial of Service Attack, flooded our servers at approximately 2 p.m. At this time the source of this attack is not fully known."

It's unclear if recall advocates have the support to make their effort a success. Republicans are already preparing to investigate and challenge any allegations of fraud related to the petition drive.

The last governor to be recalled was Gray Davis, the Democratic governor of California, in 2003.

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