Microsoft has come crashing into the public debate on the SR-520 bridge, taking out a full-page ad in The Seattle Times and trotting out general counsel Brad Smith in favor of the consensus solution for the bedeviled project. That stand, in turn, takes direct aim at Mayor Mike McGinn, who has been flirting with a last-minute, last-ditch, transit-tilting solution.

Understandably Microsoft, with thousands of commuters using the congested floating bridge, has been a mighty force in Olympia for a solution, even as debate has dragged on for a decade. But the company normally plays a back-room game, applying forceful pressure on legislators and the governor, rather than saying much in public, lest it seem to be big-footing the process. (Boeing long ago perfected that strategy, for the same reasons.)

That Microsoft has come forth publicly, even as consensus seems very strong for the current leading proposal (a new bridge with four general-purpose lanes and two dedicated to buses and carpools), indicates the urgency and perhaps the fragility of the coalition. A particular worry for these advocates is Speaker Frank Chopp, who has sided with McGinn and Montlake on this issue, and is famous for his no-fingerprints executions of legislation he does not like. In turn, Democrats on the Eastside, a key part of the Chopp majority on the House, do not want to risk deeply angering Microsoft, a key player in legislative elections.

Here's some of what Microsoft said in its ad this morning:

We've known for many years that the aging Highway 520 floating bridge must be replaced — to ensure the safety of the public and a reliable transportation system for the region. After 13 years of deliberation and outreach, there is finally an agreed-upon design and funding plan for a new bridge, and the state is ready to begin construction. Contracts are in place to begin building the new bridge pontoons in Grays Harbor County. We commend Gov. Gregoire, state legislators, and local leaders who have helped bring this important project closer to reality.

While there are still some final design issues that need to be resolved with the City of Seattle, we should not let last-minute objections undermine the hard-won agreements already in place for the rest of the project. Doing so would cause yet more delay, increase the cost to taxpayers, and put this vital transportation and economic corridor at risk. The current bridge is 47 years old, and state engineers warn that it could sink in a major storm or earthquake.

What Microsoft is attacking is a proposal for reserving the two new HOV lanes for buses and rail transit only. Advocates argue that rail on the span will be necessary soon, so let's not miss the chance to design the bridge now for that desired outcome. Less overt is the desire to exclude cars (even carpools) from the new lanes, presumably forcing drivers to change their retro habits. Another big factor is the desire by nearby neighborhoods, who don't like all the off ramps from these new lanes messing up Montlake, to pour some sand in the gas tanks of the consensus version.

Barely missing a beat, Mayor McGinn returned the fire by announcing a consultant's contract to include study of incorporating light rail into the bridge. The San Francisco consultant, Nelson/Nygaard, was originally just going to look at design issues, not light rail, but now will also look at the light rail option. The city council is also underwriting the study and, according to the mayor's office, has been apprised of the wider scope.

Unleashing Microsoft is a good indicator of the stakes in this debate. As the recession lingers, business interests are getting less patient with the dark-green, de-highway agenda, even as Mayor McGinn keeps encouraging the more radical positions. Business interests are going for the kill in the legislature, while also sending an unmistakable message to the rookie mayor. That message is also a kind of overture to what may happen on the waterfront tunnel, where so far the advocates have been treating McGinn's heresy as a kind of harmless side-show. So far.

The other reason for a full-court press by those wanting to move ahead with the 520 consensus plan is to try to isolate Montlake, a neighborhood famous for getting its way in transportation issues that affect it. One rule of thumb in local politics: "The only thing certain in state politics is that Montlake will ultimately win any argument over 520." (The ultimate weapon is a lawsuit, dragging the issue on for perhaps another decade until proponents give up.) The city council knows this and is at least seeking protective cover of the new study. Legislators, already very nervous over the anti-incumbent mood, know this and will be tempted to postpone an anti-Montlake vote (or an anti-Eastside one) until after the election.

Hence the strong implied Microsoft message (with apparently more ads to come): Act now, before too many more last-minute solutions confuse the political landscape, or be prepared to face some consequences in the next election.