NASHVILLE (AP) — Faced with a potentially serious primary challenger, Tennessee Sen. Lamar Alexander was not about to sit back and wait.The former two-term governor locked down endorsements, banked more than $3 million and linked arms with popular Sen. Rand Paul of Kentucky and former GOP presidential contender Mike Huckabee. When conservative state Rep. Joe Carr announced he would challenge Alexander, the senator’s team was ready.“I learned to count in Maryville City Schools,” Alexander wrote in a recent op-ed in The Tennessean, recalling his East Tennessee hometown. “So I know that if you only have 45 votes and you need 60 senators to get something important done like balancing the budget and fixing the debt, then you have to work with other people — that is, IF you really care about solving the problem.”Call it the Revenge of the Incumbents: Alexander is the latest Senate Republican staring down a primary challenge with aggressive tactics aimed at blunting any early momentum from tea party-backed opposition.