DENVER (AP) – Several Democratic presidential candidates are running on a promise to ban fracking and stepping on unstable political ground as they do so. Must-win Pennsylvania has gotten an economic boost from fracking, the technique used by the energy industry to extract oil and gas from rock. The procedure is also vital in swing state Colorado and in the Democrats’ dream state of Texas, the headquarters of the energy industry. Fracking has become steadily less popular as worries about the side effects of the energy extraction technique have increased. It’s been linked to water contamination, air pollution and even earthquakes. But it’s also helped bring down the cost of natural gas and the use of dirtier-burning coal. Even most environmentalists have proposed regulating it rather than banning it.