Wisconsin is a doubly important state for both parties. Its 10 electoral votes are critical in the presidential election, and the last time a Republican won there was 1984.

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And the state's Senate race could help determine control of that chamber. Republicans only need a net gain of four seats (if President Obama wins reelection) to control the Senate.

Priebus said that Wisconsin is the competitive race where the GOP is strongest in its battle to win control of the upper chamber.

"I think if there's any seat right now that we're the strongest in, in America, as far as a challenge race, it's Wisconsin. I mean Tommy Thompson is the absolute perfect candidate to win that seat," he said.

And he said there were no concerns that the competitive primary — where four candidates battled it out in a tough race for the Republican nomination — would boost Democratic nominee Rep. Tammy BaldwinTammy BaldwinSenate approves Trump's Agriculture chief Lawmakers talk climate for Earth Day, Science March Trump says he supports Dem ‘Buy America’ bill MORE.

"I'm one of these guys that big, tough, primaries are good. I like it. It makes a candidate stronger."

Preibus, a Wisconsin native, also praised his longtime friend, GOP vice presidential candidate Paul RyanPaul RyanNew ObamaCare repeal bill puts centrists on the spot Ryan rejects ObamaCare subsidies in funding bill UN contacted Trump administration on ObamaCare repeal: report MORE, who polls show has boosted Romney's chances in the state.

"He tries to win the mission every day. He works hard. He waits on God's timing and really good things happen. I would say Paul is one of these win-the-day guys — set goals and meet goals," he said.

Preibus also expressed confidence Paul would give a great speech at the convention. "There's no doubt that Paul will hit it out of the park on Wednesday."