A poll released this afternoon by SurveyUSA says that only 53 percent of Seattle residents support Mayor Mike McGinn's plan to pay for a seawall rebuild by raising property taxes. The measure would need the favor of 60 percent of voters to pass in May.

But Bill Broadhead, a partner of ConstituentDynamics and a leading strategist for the McGinn campaign, released his own poll that shows 70 percent of voters would raise property taxes to build a seawall. McGinn presented that poll to the city council on Monday.

So why the chasm of discrepancy?

"They surveyed all adults instead of likely voters, and that could have a huge impact on the results," Broadhead says. The SurveyUSA methodology suggests that the first person to answer the phone—whether or not the person votes—is the one who answers the questions for the robo-poll.

Although there are 483,000 people in Seattle over the age of 18, only about 107,000 registered voters in Seattle have a voting record that indicates they are likely to vote in a special election, says Broadhead. "That is the universe Constituent Dynamics drew their sample from," he says. "We frequently see significant differences in electoral attitudes between likely voters versus the public at large."

Today's poll made another error by not mirroring the question as it would appear on the ballot, Broadhead says. SurveyUSA asked, "Do you support or oppose this proposal?" But that's the wrong question because ballots don't ask if voters "support" or "oppose" a measure. ConstiuentDynamics asked, "would you vote yes to approve, or no to reject this excess levy?" which mirrors the question on a ballot.

"We have great respect for SurveyUSA and would be most interested in their polling results if they interviewed likely voters and presented a question that more closely approximated the likely ballot title," Broadhead says.

But SurveyUSA also provided an important piece of information Broadhead didn't: Today's poll explains the total amount a property owner could pay: "That would be about $48 on a $400,000 dollar home." Broadhead's poll simply explained that the tax amounted to "approximately twelve cents per thousand dollars" of assessed value.

Which poll to trust? I'd lean toward Broadhead's results. McGinn said throughout his campaign, when Broadhead was his strategist, that polling showed he could win. Meanwhile, SurveyUSA consistently showed McGinn trailing Joe Mallahan. SurveyUSA was wrong about the mayor's race—and may be wrong about this, too.

Today's poll also reports that Seattle residents think the tunnel should be built—but they say by a 19-point margin that it shouldn't be built if Seattle has to pay for cost overruns.