Gov. Scott Walker stepped up his support for U.S. Sen. Ron Johnson over the weekend, campaigning with his fellow Republican across Wisconsin on Sunday.

Walker said he hasn't given up on GOP presidential nominee Donald Trump winning Wisconsin, but he said Johnson's Senate re-election bid is the most important contest in the state this fall.

The governor appeared alongside Johnson at campaign stops, calling Johnson's Democratic opponent Russ Feingold a liar and claiming Feingold didn't "do squat" during his 18 years in the U.S. Senate.

While Walker made time to campaign on Johnson's behalf, he didn't appear with Trump in Green Bay last week, chalking the absence up to a prior commitment. But he said he still wants Trump to win Wisconsin.

"In my mind, I'm committed from the White House, to the statehouse, to the courthouse to elect Republicans," Walker said. "(The) most important race in the state is Ron Johnson's. It will affect not only this state, but potentially this country."

Political analysts have said the Wisconsin race is one of a few that could determine partisan control of the U.S. Senate.

Johnson said no matter who is elected president, he'll try to work with that person to solve the nation's challenges.

Feingold said Saturday that Johnson and Walker are beholden to corporate interests like the billionaire Koch brothers.

"A Complete Phony"

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Johnson also used the weekend campaign swing to revive his criticism of Feingold as "a phony."

He used the gibe against Feingold in a Politico article last year and took the chance to dust it off Friday during a Milwaukee newspaper interview, and Sunday night, during a campaign stop in Waukesha, calling Feingold "a phony" several times, including when talking with reporters.

"I'm sorry. I hate to say it, Sen. Feingold's a complete phony. I'm the real deal. I'm genuine," Johnson said.

Johnson touted his private sector experience and his chairmanship of a Senate committee, while claiming a super PAC associated with Feingold spent most of the money it raised on the Democratic candidate.

The Feingold campaign responded that Johnson, "down in the polls and desperate to stay in power, has followed Donald Trump's lead and dragged his campaign into the gutter with angry rants and personal insults."

Asked to respond to Johnson's comments on Monday, Feingold said he wanted to keep things civil.

"I've not heard a lot of that kind of thing in the races I've been in in the past, but I realize when somebody's desperate, when a politician feels like he's probably going to lose, he gets a little shaky and starts doing some things that maybe he shouldn't do," Feingold said.

Feingold said he still respected Johnson.