Letter-writing campaigns are officially a thing of the past. President Obama is calling on Americans to use Twitter as a device to pressure Congress into saving middle-class tax cuts. “If there’s one thing that I’ve learned, when the American people speak loudly enough, lo and behold, Congress listens,” he said Wednesday. The president urged the middle class to tweet their support of his proposal not to raise taxes on families earning less than $250,000, with a specific hash-tag: #My2K. “If we get this wrong, the economy is going to go south,” the president said.