Hennepin County Commissioner Jeff Johnson beat out three other Republicans to capture his party's endorsement for governor on Saturday. Now he must prepare to beat three more in the state's first major contested GOP primary in two decades.

A mild-mannered attorney and veteran politician who promised he has the general election appeal to beat Gov. Mark Dayton in November, Johnson emerged victorious in a volatile contest that saw many delegates leave before it was over.

Nevertheless, a buoyant Johnson told the remaining delegates, "We are going to leave this convention ready to campaign so hard that Mark Dayton is not going to know what hit him until about the middle of December." Johnson said that with his experience in county government, he would be able to attract votes even in the DFL strongholds of Minneapolis and St. Paul.

Johnson, a former House representative, previously lost a 2006 statewide race for attorney general.

One of the candidates Johnson beat out, former House Rep. Marty Seifert, is already planning for the Aug. 12 primary. Businessman Scott Honour and former House Speaker Kurt Zellers bypassed the endorsement altogether and also are working on their primary campaigns.

While some candidates were in Rochester wooing Republican delegates, Honour headed for Duluth on Saturday, to hold a news conference with new running mate, state Sen. Karen Housley, of St. Marys Point, not far from the convention center where Minnesota DFLers were giving their official backing to Dayton for a second term.

Dayton, who won by a whisker-thin margin after a recount four years ago, said he's up for the challenge of what promises to be a brawling re-election fight.

"We're not only going to keep making Minnesota better, we're going to make Minnesota the best," Dayton said. "We're going to make Minnesota the best place to live, the best place to marry the person you love, the best place to raise a family."

Before Republicans in Rochester even got to Saturday's gubernatorial contest, they first waded through a protracted U.S. Senate endorsement process that stretched into the predawn hours Saturday, recessed briefly, then resumed midmorning. By the time gubernatorial candidates took the stage Saturday afternoon, they were faced with trying to jolt the sagging energy levels of delegates with loud music, polished videos and floor demonstrations.

Rob Farnsworth, a Hibbing special education teacher, was quickly eliminated, followed by Thompson. Seifert, who lost the 2010 endorsement to Tom Emmer, angered some delegates and party officials when he released his delegates but stayed on the ballot in what many interpreted as a failed attempt at blocking an endorsement.

State party Chairman Keith Downey denounced Seifert and one convention delegate was seen throwing a Seifert sign to the ground in anger.

Dayton in strong position

While Republicans will jostle for primary position over the next two months, Dayton comes into the race with unrivaled name recognition and a long career in Minnesota politics. He also brings a string of legislative accomplishments, including balancing the budget, raising the state's minimum wage and legalizing same-sex marriage.

But Dayton and his running mate, Tina Smith, face a daunting and uncertain five months until the election. The governor's once-soaring popularity has taken some blows lately and Republicans nationally are increasingly optimistic they will make gains in Congress and statehouses across the country.

The convention offered a ­triumphant moment for Dayton, who has had a complex and at times rocky relationship with DFLers. In 2010 Dayton was not even allowed on the floor of the DFL convention and won the primary running against the party's endorsed candidate.

This time, Dayton arrived in Duluth as the unquestioned party leader, a tough-minded chief executive who prevailed over Republicans after a bruising government shutdown. The election following the shutdown jettisoned GOPers from control of the House and Senate and put Democrats firmly in charge at the Capitol.

"Republicans said that if we raised taxes on the rich and closed corporate loopholes, we would ruin our state's economy. Instead, businesses have added 145,000 new jobs in Minnesota during the past three years," said Dayton, who accused Republicans of seething over the state's successes.

The governor called on Democrats to build on past achievements and vowed to intensify his focus on the economy, the environment and making lives better for Minnesotans.

"There can be no regression to the Republican redressing of the past," Dayton said. "There's only moving ahead, even better than before, toward the best it's ever been."

Democrats also endorsed Minnesota Attorney General Lori Swanson, state Auditor Rebecca Otto and Hopkins state Rep. Steve Simon, who won a contested endorsement fight for secretary of state.

Democrats are already building a campaign machine to help Dayton and other DFL candidates hold their seats.

Dayton has raised more than $1 million and is building a formidable operation, with six district coordinators already anchored around the state. The Republican candidates will also be forced to compete for donors.

Alliance for a Better Minnesota, an independent group largely credited with helping Dayton win his first term, is hammering GOP candidates with an almost daily barrage attacks on past votes or business ventures.

The group is expected to unleash a barrage of television advertisements later this summer and has already spent more on advertising than all of the GOP candidates combined.

"Are we going to fight hard for this? You're damn right we are. We have to," said Ken Martin, chairman of the DFL Party. "If any Democrats in the state think we are going to coast on this, they are fooling ­themselves."

Staff writers Abby Simons and Rachel E. Stassen-Berger contributed to this report. baird.helgeson@startribune.com 651-925-5044