Democrats and Republicans along Massachusetts’ South Shore are girding for a rare competitive primary and general election in the wake of Rep. Bill Delahunt’s (D) decision not to seek an eighth term in November.

Delahunt’s move was not a huge surprise; he’d been signaling a possible retirement since earlier this year and did not raise much money in the final quarter of 2009. And Sen. Scott Brown’s (R-Mass.) victory in January’s special election and his strong performance in Delahunt’s 10th district in particular had generated interest in the seat from a handful of seasoned GOP officials.

The district is the most Republican in the liberal state, but it still favors Democrats. Delahunt won each of his re-elections with more than 60 percent of the vote, and President Barack Obama won the district by 12 points in the 2008 presidential race. But as Brown’s surprise victory proved, the state’s large bloc of independents is disgruntled and prepared to side with a Republican candidate in the right circumstances.

The Republican primary is likely to come down to a face-off between state Rep. Jeffrey Perry and former state Treasurer Joe Malone. State Sen. Robert Hedlund ruled out a run Thursday night in the wake of Delahunt’s announcement.

The Democratic contest is wide open, particularly given the recent decision by Joe Kennedy III, grandson of Robert F. Kennedy, to pass on the race.

Several Democrats had already been expressing their interest based on the expectation that Delahunt would retire, including Norfolk District Attorney William Keating. State Sen. Robert O’Leary would be another leading contender for the seat.

Other Democratic names that have been floated include wealthy insurance executive Philip Edmundson, state Reps. James Murphy and Ronald Mariano, and Ian Bowles, secretary of Massachusetts’ Executive Office of Energy and Environmental Affairs. Bowles ran against Delahunt in the 1996 open-seat primary.

Perry is already in the race on the GOP side, and Malone plans to officially announce his campaign on March 21 in Plymouth, according to an informed Republican source. Malone told Roll Call that he is already in the process of organizing a campaign, including “setting up headquarters, setting up the Web site, hiring the staff and the consultants and so forth.”

Longtime Massachusetts Democratic consultant Dan Payne said Malone “probably has the best name recognition and can probably raise the most money” of the two Republicans, though he noted he has alienated some in the state’s GOP establishment in the past. Malone endorsed and campaigned for former New York Mayor Rudy Giuliani (R) for president over former Massachusetts Gov. Mitt Romney (R) in 2008 and took on then-Gov. Paul Cellucci (R) in a bruising primary in 1998.

Also, the state treasurer’s office was rocked by an embezzlement scandal under his watch in the late 1990s, though he was never implicated in any wrongdoing.

On the Democratic side, Keating has raised his profile over the last month by taking a vocal stand on the Amy Bishop controversy. Bishop, the neurobiologist who is accused of gunning down three of her colleagues at the University of Alabama earlier this year, escaped conviction in a shooting in the 1980s under the watch of then-District Attorney Delahunt. Keating has criticized that response and called for an investigation.

Delahunt told the Boston Globe in an interview published Thursday that politics had nothing to do with his decision to leave Congress.

“Life is about change. I think it’s healthy. It’s time,” he told the Globe. He will address supporters in his hometown of Quincy on Friday afternoon.

“Although Delahunt said it wasn’t a factor [in his retirement], it’s got to be,” Payne said of the Bishop flap, “especially if Keating were to get in a primary.”

However, Keating has one potential roadblock: he does not live in the district.

O’Leary has a strong base in Barnstable, one of the district’s main population centers. But Republicans are already prepared to tag him as a tax-raiser, noting that he voted last May for a sales tax increase, an amendment that passed with a veto-proof 29-10 majority in the state Senate.