WASHINGTON (Reuters) - President Barack Obama will use a tour of three election battleground states next week to push Congress to prevent the doubling of interest rates on federal student loans -- an effort aimed at re-energizing his support among younger voters. Obama will speak at universities in North Carolina, Colorado and Iowa, the White House said on Friday. All three states are considered critical to his re-election chances, and the youth vote is one of his key constituencies. The two-day trip is part of a campaign by the Obama administration to get Congress to extend low interest rates on...