Massachusetts lost residents for the second year in a row, new federal Census estimates show, underscoring an accelerating population shift from the Northeast to the South and West that threatens to erode the state's political and economic clout. Only two other states, along with the District of Columbia, lost population from July 1, 2004, to July 1 of this year, according to US Census estimates released yesterday. The Bay State lost about 8,600 residents, or .1 percent of its population, according to the estimates. If the trend continues, specialists say, the state will face serious consequences: fewer seats in Congress,...