URBANA — Six-term Illinois Congressman Tim Johnson abandoned his re-election bid on Thursday, leaving his fellow Republicans scrambling to find a replacement and fueling Democrats' hopes of picking up seats and regaining control of the U.S. House.

Johnson, who built a reputation for breaking with his party on certain issues, said he plans to retire when his current term ends. He cited family obligations for the decision.

"One of my grandsons is 2 years old. I have seen him for a total of about 10 minutes. I have another who asked me not long ago if I was ever going to come to one of his ballgames. I didn't have an answer," Johnson said.

The 65-year-old made the announcement in a statement shortly before holding a news conference in his hometown of Urbana, just weeks after his easy primary win over a Republican challenger.

The news of Johnson's decision baffled officials in both parties, and left the state GOP scrambling to find potential replacement candidates to run in his new western Illinois district.

"He's been in public office for a long, long time. The suddenness is a little striking," said Illinois Republican Party Chairman Pat Brady. "We're sad to see him go because he's such an effective campaigner."

Brady and other Republicans said they think Johnson is simply tired. With a reputation for being health conscious — Johnson was an avid swimmer, treadmill user and made no secret of his low-fat diet — there was nothing to indicate he might be ill.

"He's a prodigious worker. He's probably burned out," Brady said.

Johnson was elected to the General Assembly in 1976 and to Congress in 2000. He earned a strong reputation for reaching out to constituents over the years, making up to 100 calls a day. He had continued the same approach throughout the primary election with strong fundraising and had even rented an apartment in Litchfield, which was unfamiliar territory to him on the new congressional map.

"There's no one more energetic guy than that guy," said Montgomery County Republican Central Committee chairman Roy Hertel, who also added that Johnson was not afraid to break with his own party. "He's been his own person. I don't think he's marched to anybody's drum."

Last year, Johnson called for an end to the wars in Iraq and Afghanistan, and last month, he endorsed Texas Rep. Ron Paul in the Republican presidential race. He was Illinois' only Republican member of Congress who didn't object to the new state congressional map, which Democrats drew to favor themselves.

The newly drawn district was to strongly affect Johnson. Just a small portion of his old eastern Illinois district was drawn into the new 13th District, a swath of west central Illinois that stretches to include parts of 14 counties

His seat had widely been considered safe by national Republicans. In fact, Johnson felt so secure that he stayed in Washington on election night last month with no party planned in Illinois. Come November, he would have faced perennial candidate David Gill, an emergency room doctor who Johnson easily defeated three times in previous elections.

Democrats believe Johnson may have been more vulnerable this time around. He would have had to introduce himself to many new constituents in a more Democratic-friendly district, and for the first time a decade, he faced primary opposition.

Gill, who narrowly won a primary contest against a Democratic candidate backed by U.S. Sen. Dick Durbin, said he was in a better position than ever to win the seat, considering the new district.

"It was going to be a rough battle," Gill said.

The Democratic Congressional Campaign Committee, which believes the party could pick up five seats in Illinois, identified Johnson's district as one to watch. Illinois saw its first Republican congressional delegation in years after a 2010 Republican surge sent five new GOP freshmen to Washington. But Republicans have already lost one seat this year; Longtime U.S. Rep. Don Manzullo lost a primary battle with first term U.S. Rep. Adam Kinzinger.

The National Republican Congressional Committee declined to comment on Johnson's retirement ahead of his announcement. But officials have said that Gill, who is pro-choice and advocates for comprehensive immigration reform, is too liberal for the district.

The process to replace Johnson will take shape over the coming weeks. A number of candidates have been mentioned as possible contenders including Rodney Davis, an aide to U.S. Rep. John Shimkus, and Johnson's former chief of staff, Jerry Clarke.

After those who are interested step forward, county chairmen from the 14 counties in the new district will vote, but before that, the chairmen face their own election later this month, Brady said.