Deval Patrick and Charlie Baker are feuding over the Green Line extension.

By George P. Hassett

The long-delayed Green Line extension through Somerville became a potential political wedge issue this week after Republican challenger Charlie Baker criticized Gov. Deval Patrick for the project’s cost overruns.

The cost of the Green Line extension has risen from $560 million in 2007 to $954 million.

“This is how Governor Patrick has conducted business in Massachusetts during the past four years,” Baker said in a prepared statement. “Costs have gone up on his watch on everything from a cup of coffee to renewing your license to property taxes because Deval Patrick cannot manage state government. In Deval Patrick’s mind, it’s always easier to raise costs than make the tough decisions.”

Charlie Baker

In 1990, state officials promised city residents they would complete the extension through Somerville and into Medford as a way to offset air pollution caused by the Big Dig. In 2005, the Conservation Law Foundation (CLF) sued the state because it had not taken the necessary steps to complete the project on time. In November 2006, CLF and the state settled and agreed on a binding commitment to complete the project by 2011. That commitment was pushed back to 2014. A further delay is expected after activists and state officials clashed on where to locate a maintenance facility for the trains.