PORTLAND – Two of Maine’s largest power-distribution companies, Central Maine Power and Maine Public Service, are proposing major upgrades aimed at preventing blackouts, bringing as a side benefit thousands of construction jobs.

The two companies want to spend $2 billion to upgrade their transmission systems in projects that would broaden hundreds of miles of existing power line corridors and create new ones from Canada to Maine. The two-year project, which would begin in the winter of 2010, would create an estimated 8,000 jobs.

CMP wants to overhaul the state’s bulk power grid after a study that said the 37-year-old existing bulk transmission system would soon be inadequate to carry Maine’s power needs.

CMP would spend $1.4 billion to widen the existing power line corridor along a 370-mile path from Orrington, just south of Bangor, to Newington, N.H. CMP also would add 345,000-volt transmission lines and other equipment.

CMP and MPS would spend $625 million to build a new 200-mile corridor with a 345,000-volt transmission line from central Maine to northern Aroostook County to connect with wind farms being developed in that area.

Without the upgrades, Mainers can expect possible power disruptions in the future, company officials said.

“We see 2012 as a critical year if loads continue to grow,” said CMP spokesman John Carroll.

The permitting process before the Maine Public Utilities Commission has drawn dozens of participants, including municipalities, business groups, homeowners, environmental agencies and the state Public Advocate’s office. Approval would clear the way for the power companies to take the land by eminent domain.

The bulk transmission system carries electricity from generating plants to areas where large amounts of electricity are needed. From there, distribution lines carry the electricity to where it’s used.

Maine’s population and its sources of electricity generation have shifted since the system last was upgraded, making the improvements necessary, CMP says. More people have moved into southern Maine, while power generators have become more distant and dispersed, especially with the closing of the Maine Yankee atomic plant.

Central Maine Power is asking ISO New England, the private body that oversees New England’s electrical grid, to help share the cost. If that request were approved, Maine ratepayers would pay for only 8 percent of the project.