Republicans are casting the intense battle between Senate hopefuls Gabriel E. Gomez and U.S. Rep. Edward J. Markey as an early referendum on the scandal-wracked Obama administration, as Democrats scramble to widen the congressman’s lead in the polls just 10 days before the election. “This is definitely a canary-in-the-coal-mine race,” Republican consultant Ron Bonjean said. “You’re seeing the polls dropping and a Republican in Massachusetts once again having a very decent shot at winning the election. It’s a shot across...