Bigger and bigger data centers are leading to proposals for massive electric power transmission lines, sometimes across hundreds of miles. These high-voltage power lines cost tens of billions of dollars a year and are the latest front line in the battle over tech giants' massive operations. Artificial intelligence advances are seen by President Donald Trump as critical to the nation’s economic and national security. But their energy needs are threatening to overwhelm the power grid and the transmission expansion is drawing opposition from landowners, conservationists, local officials, consumer advocates and states.