The first letter arrived at the Republican’s campaign headquarters in a suburb of Denver, Colorado. A second letter was later reported at a McCain office in Manchester, New Hampshire. Staff members were sent to medical facilities for treatment under quarantine while FBI and Secret Service agents joined hazardous materials experts at both scenes.

Republican Party’s challenges in the nine-state Northeast region are a reflection of what the party faces across the country as it is being forced to defend dozens of Congressional seats that are now considered competitive at a time when the party has limited financial resources, political analysts said.

In the wider context of a resurgent Democratic Party, which is gunning to push its slim 51 to 49 Senate majority as close as possible to a filibuster-proof 60 seats, Senator John E. Sununu, whose seat has been in Republican hands for 30 years, faces a grueling rematch against former Governor Jeanne Shaheen.