Americans for Prosperity, part of the Koch Brothers conservative political network, is announcing a $4.5 million ad buy today to pressure three Democratic senators to support the GOP tax cuts plan. The senators: McCaskill (MO), Baldwin (WI) and Donnelly (IN).

Why it matters: The tax push won't lack for funding — AFP says this is just the first wave of ads. And with Republicans like Bob Corker and Rand Paul sending warning signs about their positions on the plan, conservatives are hoping to get a couple Democrats on board.