Map changes incumbents’ terrain

Within hours of the announcement, incumbent William R. Keating, a Quincy Democrat, declared that he will move to his summer home in Cape Cod, in order to avoid running against fellow Democrat Stephen F. Lynch of South Boston.

In another major overhaul, the plan would create a new seat representing Cape Cod, the South Coast, and the coastal sections of Plymouth County.

The new map would also expand the only district in the state that has a majority of nonwhite residents, now represented by Michael E. Capuano of Somerville, extending it from Somerville and Everett, through Boston, and down to minority precincts in Milton and Randolph.

Meanwhile, the two Western Massachusetts districts would be completely revamped, with the Berkshires set to be represented by Richard E. Neal, a Springfield Democrat.

Through its efforts, the redistricting committee has backed up promises to untangle what has become a circuitous map of meandering districts created over decades to satisfy partisan yearnings for power.

“We didn’t tinker on the edges here,’’ said the House cochairman, Michael J. Moran of Brighton. “We made some serious decisions.’’

In fact, revamping is very likely to prompt several primary battles for incumbents and even give Republicans a serious shot at gaining a foothold in the all-Democrat delegation.

The most politically explosive move involves the merger of the districts of two Democratic incumbents, Lynch and Keating, whose district currently extends from Quincy to Cape Cod. Keating decided it was not worth a head-to-head battle.

“I have decided to run in the new Ninth Congressional District, where we have owned a home for 17 years,’’ Keating announced late last night.

It is not unusual in Massachusetts for political figures to relocate to run for Congress. Keating moved two years ago from Sharon to Quincy to campaign for that seat. In 1980, Barney Frank, a state representative from the Back Bay, moved to Newton to seek that congressional seat.

The two cochairmen of the redistricting committee acknowledged they have made major changes to the district boundaries that could rattle incumbents, several of whom could face districts with vastly different populations and interests than the ones they now represent.

“Everybody needs to digest this,’’ said Senator Stanley C. Rosenberg of Amherst.

With veteran US Representative John Olver of Amherst retiring, some expected the committee to merge his communities into surrounding districts to avoid crunching two incumbents. But Moran has said the creation of a South Coast and Cape Cod district was a priority.

“We’ve drawn up what we believe . . . is an accurate reflection of where the people of the Commonwealth are,’’ Moran said.

In some regards, the redistricting process in Massachusetts is easier than it is in other states, because the Democratic political leadership wants to retain the party’s advantage.

But “this is clearly not drawn for incumbents,’’ Moran said. “If it was, it would look a little different.’’

Even Republicans, who have been shut out of congressional seats for years, could have a shot of winning in the new configuration. That is particularly true in the North Shore district represented by John F. Tierney, a Salem Democrat. Former Senate minority leader Richard R. Tisei, a socially liberal Republican, said yesterday he plans to challenge the eight-term congressman.

Tisei said the addition of Billerica and Tewksbury, towns that have voted Republican in recent years, could help the party defeat Tierney. “Those are very fertile territories,’’ he said.

In the most western district of Massachusetts, Neal, a twelve-term veteran, will face a primary challenge from Andrea F. Nuciforo Jr., the former state senator from Pittsfield.

Minority groups, which had successfully sued the state over redistricting in the past, hailed the plan yesterday as a success because it increases the percentage of minorities in Capuano’s district.

“We’re ecstatic about this,’’ said Kevin C. Peterson, executive director of the New Democracy Coalition, a minority voting rights group that includes the Urban League, the NAACP, and the Boston TenPoint Coalition. “They gave us exactly what we asked for.’’

Peterson said the plan will increase the percentage of minority residents from 52 percent minority to nearly 57 percent, making it more likely that the state could send a person of color to Congress within five years.

Capuano said he is not worried about a potential challenge. “I have never thought people vote just along racial identity lines,’’ he said. “If that’s the case, I would never have been elected.’’

In the southeastern part of the state, a new congressional seat could draw a flood of interest.

“An open seat in Congress, that’s like a want ad,’’ said Mayor Scott W. Lang of New Bedford, who is leaving office after his term expires at the end of the year. “You’d have an awful lot of people who get up in the morning look in the mirror and see a congressman.’’

Lang mentioned C. Samuel Sutter, the Bristol district attorney, as a potential Democratic candidate, and Thomas M. Hodgson, the Bristol County sheriff, as a potential Republican contender. Lang said he would not rule out running, but has no immediate plans beyond returning to his law practice.

In the final hours of negotiation, the redistricting committee backed off an earlier House plan, that would have hurt the delegation’s only woman, Niki Tsongas, by removing the city of Lawrence from her district.

Another incumbent, Barney Frank, will keep his base of Newton and Brookline, while losing Democratic strongholds in the New Bedford area and in Buzzards Bay communities where he posted large winning margins.

Moran and Rosenberg defended combining Fall River with Boston suburbs on the grounds that Frank’s seniority as ranking member of the House Financial Services Committee is important for the state.

The plan must now be approved by the full House and Senate before the Legislature’s session ends Nov. 16 and then be signed by the governor.

But Senate minority leader Bruce Tarr, a Gloucester Republican, said the public should have at least two weeks to review it.

“This map will involve some of the most important redistricting decisions that have been made in the Commonwealth in decades,’’ he said.

Billy Baker of the Globe staff contributed to this report. Frank Phillips can be reached at phillips@globe.com.