State Sen. Ed Murray, City Councilman Bruce Harrell and former City Councilman Peter Steinbrueck are among the candidates voters are choosing from Tuesday.
McGinn's opponents say his governing style is divisive and that McGinn has alienated business groups and the state Legislature. McGinn, an attorney and former Sierra Club leader, points to the city's improving economy, increased spending on education programs and his support for more transit options.
The top two finishers in the primary advance to the general election.
McGinn narrowly defeated businessman Joe Mallahan in the general election four years ago. McGinn's predecessor, Greg Nickels, finished third in the 2009 primary.
Known for often riding his bicycle to campaign events, McGinn as mayor pushed for more public transportation options. He opposed a $3.1 billion tunnel replacement for an elevated state highway along Seattle's waterfront that Nickels had advocated and the state Legislature approved. McGinn, who wanted to tear down the Alaskan Way Viaduct and disperse traffic on surface streets, pushed a 2011 citywide referendum on the tunnel project. Voters supported the tunnel, dealing McGinn a significant political blow.
He also vetoed a measure passed by the City Council that would've allowed police to cite aggressive beggars. The city's downtown businesses supported the panhandler ordinance, saying people felt unsafe. McGinn said the idea unfairly scapegoated homeless people and opponents like the American Civil Liberties Union said the behavior being targeted was already illegal. He was criticized in 2012 for fighting federal officials who sought greater oversight of the city's police force. Ultimately, Seattle agreed to an independent monitor of the city's police department as part of a deal with the Justice Department following a report that found officers routinely used excessive force.
More recently McGinn, citing concerns about pollution, has opposed plans that would have Washington state host export terminals and tracks for trains hauling millions of tons of coal from Montana and Wyoming and destined to Asia.
Murray, who led the state Legislature in passing a gay marriage bill last year, has said he would take a more collaborative approach to dealing with transportation and public safety issues. Steinbrueck, an architect who served on the City Council from 1997 to 2007, says the city's neighborhoods have been neglected. Harrell has stressed community service and education in his campaign, saying graduates of Seattle public schools should get one year of free tuition at local colleges.