President Obama is taking heat from liberals for giving the go-ahead for Democratic-affiliated super PACs to get up and armed. On Tuesday, members of the administration tried to explain the decision to reporters, repeatedly raising the specter of a weekend Koch brothers conference at which big-shot donors pledged $100 million to defeat Obama. Obama advisers fear that number could grow to half a billion dollars for Republican attack ads. “We recognize the political reality under the law we’re living in right now, and we need to be realistic about that,” one official said. Obama still opposes the Supreme Court decision that gave birth to super PACs, and supports legislation to reform campaign finance, but his advisers made clear that they would have to play hardball to contend in November.