PORTSMOUTH, New Hampshire (CNN) -- Republican Sen. John McCain, who has been campaigning for president for months, officially announced his run for the White House Wednesday, stressing the need for Democratic-Republican unity.

"We have common purposes and common challenges, and we live in momentous times. This election should be about big things, not small ones. Ours are not red state or blue state problems. They are national and global," he said at a campaign rally in Prescott Park.

"Even in the heat of a campaign, we shouldn't lose sight that much more defines us than our partisanship; much more unites us than divides us," he said.

"I must seek this responsibility for reasons greater than my self-interest; and am determined to use every lesson I've learned through hard experience and the history I've witnessed, every inspiration I've drawn from the patriots I've known."