It's not that it's impossible, but that it's just not cost-effective, and even if it does happen, it's not happening at a scale that would affect even a statewide election let alone a national election. Why go through all the effort (and risk) to modify Joe Smith's vote from Team A to Team B when Team B can just buy advertisements, write newspaper op-eds, spam social media, saturate the news cycle, take over talk shows, and otherwise propaganda thousands of Joe Smiths across the whole region into voluntarily voting for Team B?