"This campaign is about what is the vision for our future," he said. "We need to earn the future we want and we need to do it together."
The former Sierra Club chairman is the founder of the Seattle Great City Initiative and is known for his environmental advocacy.
Like Nickels, McGinn wants more buses for Seattle to deal with overcrowding.
"If you're going to have a bus lane, you better have enough buses running down it," he said.
McGinn reiterated his support that the Alaska Way Viaduct be replaced with surface streets. Voters don't want to spend $2 billion for a tunnel, he said. Back in 2007, Seattle voters roundly rejected the deep-bore tunnel in an advisory vote, but Nickels announced in January that the viaduct would be replaced with a tunnel, if he can find the money to do so.
"When you ask the voters what they think, then you better listen to them," McGinn said.
Addressing Seattle's lagging infrastructure, he argued that Seattle Public Utilities should be installing fiber optic networks because private companies tend to focus only on the wealthiest neighborhoods.
McGinn said he also supports district-based Seattle City Council elections, and like Nickels, he's for making Mercer Street more visually appealing.
Others considering a run against Nickels are former Seattle City Council member Peter Steinbrueck and former NBA Sonics player James Donaldson.