WASHINGTON — Just six months ago, Keystone for many Americans was the state nickname for Pennsylvania. But now Keystone, the Canadian pipeline, has become a centerpiece of the Republican economic and political agenda, and the party’s preferred truncheon against President Obama. 

On the airwaves, on the campaign trail and in both chambers of Congress, Republicans are relentlessly pushing for an expansion of the pipeline known as Keystone XL and criticizing Mr. Obama’s decision to reject the project for now, forgoing thousands of pipeline jobs. Democrats, increasingly worried that the pounding on the issue will detract from their own message against income inequality, are looking for ways to defuse it.

Republicans are framing Keystone as an urgent jobs and energy project at a time of high unemployment and creeping gasoline prices, and trying to portray Mr. Obama as giving in to hard-left environmentalists in an election year at the expense of addressing both.