The retirement announcement Wednesday of veteran Rep. David Obey (D) sent political shock waves through Wisconsin and prompted Democrats to immediately begin weighing how they will defend a district Obey dominated for more than four decades.

“This is just unbelievable,” said state Rep. Mary Hubler (D), who has represented part of Obey’s northwestern 7th district for nearly three decades. She said she was “shocked” by the news and added Obey didn’t mention anything about his political plans when they spoke Monday.

As surprised Democrats spent the day praising Obey’s long House service and mulling potential successors, Republicans were promoting their prospects for winning a seat that was already on their radar screen.

Obey’s retirement immediately enhanced the profile of Sean Duffy, a county prosecutor, former lumberjack champion and “The Real World” reality show contestant whose long-shot-but-determined challenge to the veteran Congressman had already attracted national attention.

“There is no question that David Obey was facing the race of his life and that is why it is understandable that the architect of President [Barack] Obama’s failed stimulus plan has decided to call it quits,” said Ken Spain, National Republican Congressional Committee communications director.

Obey rejected that argument — and suggested a district he almost always won by handsome margins would not elect Duffy this fall.

“Let me put it this way, I’ve won 25 elections — does anybody really think I don’t know how to win another one? Or for that matter has anybody ever seen me walk away from a fight in my life?” Obey said after a press conference announcing his decision. “The fact is, there isn’t a chance of snowballs in Hades of that progressive Congressional district electing someone who is a poor imitation of George Bush’s policies on a bad day.”

Democratic strategists said the 7th district usually votes for their candidates in statewide elections. It gave President Barack Obama 56 percent of the vote in 2008.

“Let’s be very clear: This is a Democratic district that has been in Democratic hands that has elected Democratic candidates up and down the ticket for generations,” said Mike Tate, chairman of the Wisconsin Democratic Party.

Just who will carry the Democratic banner in Obey’s absence is an open question that party activists only began pondering Wednesday. There is plenty of pent-up ambition among would-be successors in a district that hasn’t hosted an open-seat race since Obey was first elected in 1969.

“This will have a domino effect, as it always does when somebody after 40 years hangs it up,” Hubler said. “I’m sure there are people who have been waiting in line for their chance, and this is it.”

The long list of potential Democratic candidates that emerged in the immediate aftermath of Obey’s announcement included state Senate Majority Leader Russ Decker, state Sens. Julie Lassa and Pat Kreitlow, state Rep. Donna Seidel and Wausau attorney Christine Bremer.

Republican state Sen. Dan Kapanke, who is challenging Rep. Ron Kind (D) in the adjacent 3rd district, said Decker would be a “formidable foe” if he sought the seat and noted his long tenure in Madison. A spokeswoman for Decker said he is undecided on the race.

Bremer, a former president of the Wausau School Board, said, “I won’t rule anything out, but today is a day for Dave Obey.”

Lassa also said that speculation about her political future should wait until after appropriate tributes to Obey.

“There will be a number of us, after today and getting over the initial shock, who will be thinking if we want to take on that challenge and represent the district,” she said.

Seidel said that “no doors are completely closed.” Kreitlow didn’t respond to a request for comment.

One Democrat who took himself out of the running was state Sen. Robert Jauch, who noted he will be 65 this year — just a few years younger than the 71-year-old Obey, for whom Jauch worked in the 1970s.

Jauch said he was “confident there will be a credible Democratic candidate who will receive widespread support” in what he described as “a working-class Congressional district.”

But the list of potential Democratic candidates almost certainly will be winnowed in the coming days, in part because Wisconsin has a late Sept. 14 primary — seven weeks before the Nov. 2 general election — and Democrats will want to rally behind one candidate.

“I don’t expect to have a competitive primary up there,” Tate said. “I think that the Democrats will get together and have a conversation and that there will be a decision reached as to who’s the strongest candidate among themselves.”

Democratic officials think Duffy is beatable but are mindful he has a big head start in the now-open district. Duffy began campaigning last July and reported $339,000 in the bank at the end of March, and he should be able to accelerate his fundraising now that Obey is not running.

Duffy has the backing of many members of the local GOP establishment and is the preferred candidate of the NRCC. His lone primary opponent is Dan Mielke, a farmer and businessman who won 39 percent of the vote in 2008 as Obey’s Republican opponent.

Duffy said that his strategy wouldn’t change with Obey out of the race. “I wasn’t running against Dave Obey; I’m running on a set of principles and ideas and a certain vision for our country,” he said.