Former Gov. Tommy Thompson told a Wisconsin radio station this morning that he filed the necessary paperwork to officially begin his Senate primary campaign.

“I’ve filed my corporation documents today, which allows me now to build my organization, raise money so I can go out there and tell the story,” the Republican told Milwaukee’s WTMJ.

He added, “An announcement will be coming very soon … we’re doing it in steps because that’s what the federal law requires.”

Thompson announced campaign co-chairmen last month and has Darrin Schmitz, one of the top consultants in the Midwest, under contract.

Thompson will likely have to defend his conservative credentials against Republican primary opponents, which currently include former Rep. Mark Neumann and Wisconsin Speaker Jeff Fitzgerald. Neumann is backed by the anti-tax Club for Growth, which has already gone on the air against Thompson.

Rep. Tammy Baldwin is the likely Democratic nominee and is expected to have a national fundraising network at her disposal to help keep the seat of retiring Sen. Herb Kohl in Democratic hands.

Roll Call Politics rates this race a Tossup.