Mike McGinn wants you to know he's more than just the anti-tunnel guy.

His opposition to the deep-bore tunnel plan served as a great wedge to get him through the primary, but there is a big danger it could backfire on him in the November election.

I've had the impression (from comments by readers and in and talking to folks around town) that there is a sizeable bloc of Seattle that might prefer to vote for McGinn, because he has been involved in local politics and civic issues in a way Joe Mallahan has not.

But some of those voters are scared at the notion of tearing down the Viaduct without replacing at least some of its capacity via the tunnel. Or they're simply sick and tired of the issue and don't want to reopen a debate that has lasted eight years.

You've seen this play out in recent days as major business and labor groups have fallen in line behind Mallahan -- a guy they just pretty much just met. If the tunnel weren't an issue, I think this could have played out differently, especially for unions that attacked Mallahan before the primary.

With all this as a backdrop, McGinn and Mallahan squared off today in a small debate sponsored by the Queen Anne Chamber of Commerce.

I asked McGinn before the debate whether he was worried about losing some people solely due to his tunnel stand. He's clearly thought about that. "I would ask them not to be one-issue voters," he said.

Mallahan certainly sees an advantage in the tunnel issue now. Like he did during their first debate, Mallahan pounced on McGinn's anti-tunnel stance, saying it was irresponsible to "conduct an experiment" that would force tens of thousands of cars onto city streets. He implied it could harm business in an already down economy.

McGinn didn't back away from his tunnel stance -- he called the plan irresponsible and said it would put Seattle taxpayers on the hook for the all-but-inevitable cost overruns.

But McGinn did ask the crowd to think of the tunnel as just one issue facing the next mayor.

"Like all political issues, it will come and go," he said.

McGinn said voters should also consider the "values" he's displayed by his involvement in local politics over the years -- as a leader in the Sierra Club, Greenwood neighborhood and in local campaigns for parks and transit.

"You know these are my values because of what I've chosen to do over the years," he said.

It was an indirect poke at what continues to be Mallahan's biggest weakness in the campaign: his almost complete lack of a civic or political record before running for mayor.

Mallahan responded somewhat uncomfortably in his own closing remarks.

"I think that's fair. A leader's values are important," he said, saying he and his wife "have given back in quieter ways."

"I'm still not comfortable listing the 15 ways I've been involved" in the community, Mallahan said.

So he didn't list any.

Instead, Mallahan talked about his endorsements from unions, his "excellent" rating from the Municipal League, and how he is both a successful executive at T-Mobile and a "social justice Democrat."

I'll be curious to see how this plays out in the coming weeks.

Can McGinn convince a majority of voters either that he's correct about the tunnel, or that they should trust him even if they're not sure?

Or will Mallahan turn what seems to be default support from some pro-tunnel interests into a broader, enthusiastic campaign that convinces people he's got the management chops needed at City Hall?