Collins is the only Republican senator up for reelection in 2014 that comes from a state won by President Obama in 2012.
Poll: Collins favorite for reelection, but GOP in trouble if she retires
But if Collins is not the Republican nominee, Republicans will likely struggle to retain the seat. If former Secretary of State Charlie Summers — a likely Republican alternative — ran against either Michaud or Pingree, the incumbent members of Congress would have a decided advantage. Michaud leads Summers 57-32 percent, while Pingree holds a 50-39 percent edge.