If today's rules had applied in 1987 when voters first went to the polls to consider a half-cent sales tax to help pay for transportation projects, the measure would have failed by a double-digit margin.

Today, in order to pass Proposition A -- the measure that will ask voters this November to extend the transportation tax that voters approved nearly 20 years ago -- a simple majority simply won't do.

When the tax, known as TransNet, was on the ballot in 1987, the 53 percent approval it garnered was enough to pass it.

But a state initiative passed by voters eight years ago that restricts how taxes can be raised means that this November, 66.6 percent of voters will need to approve the 40-year tax extension.

The San Diego Association of Governments, the regional planning agency that is pushing for the extension, may push for changes to that threshold if the measure fails, said Garry Bonelli, association spokesman.

Backers concede that the two-thirds requirement presents challenges, especially when not everyone is on board. Republican legislators have skewered the plan.

The $14 billion in revenues expected from the extension will be split about evenly among freeways, local roads and public transportation projects. The current tax expires in 2008.

Critics, including county Supervisors Bill Horn, Dianne Jacob and Pam Slater-Price say more of the revenues -- 50 percent -- should go to freeways, which carry the bulk of commuters.

But proponents say that polls so far show more than 70 percent of voters would support the measure-- more than enough needed to get it passed. And they say that getting voters to approve an extension is easier than getting them to approve a new tax.

Critics have said that convincing voters that the plan is flawed will be easier because such a large percentage of the vote is needed for approval.

"That makes it much to the advantage of those of us who are willing to fight to see it not be extended," said Jerry Harmon, a spokesman for a group called Traffic Relief is Possible that opposes the measure.

Harmon served on the Escondido City Council from 1974 to 1998. Before leaving office, Harmon and his supporters ran a successful campaign to pass Proposition S, which reduced the Escondido City Council's ability to make land-use decisions. That same year, he fought an unsuccessful campaign against Bill Horn for the 5th District county supervisor's seat.

Harmon said with the well-oiled campaign that he expects to see in favor of the extension, his group will need others to speak out against the measure.

The measure that requires a higher approval threshold for some tax initiatives was Proposition 218. Passed in 1996, it requires that 66.6 percent approval for a tax assessment to pass. General taxes that are not directed to certain projects or for a specific use can be approved with a simple majority.

In that vein, the proposition is "counter-intuitive," Bonelli said. Voters can hold elected officials more accountable for the way they spend taxes directed at specific needs, he said.

If tax measures such as TransNet continue to fail after making what would otherwise be a strong showing at the polls, there will probably be challenges to Prop. 218, Bonelli said. The association board has already talked about the need to work with state legislators to get the threshold moved back to 55 percent or 50 percent plus one if TransNet fails.

Two years ago, similar measures failed in Fresno, Madera, Merced and Solano counties, despite having received more than 50 percent of the vote. Three of the five ballot measures were extensions; two were proposed new taxes. Only Riverside County succeeded in getting the two-thirds needed to extend its existing half-penny sales-tax for 30 years, beginning in 2009.

Polls commissioned by the association in July 2003 and March 2004 pegged the support for extending TransNet in San Diego County above 70 percent, Bonelli said. The one taken last year, for instance, showed 52 percent of respondents strongly supported the tax extension and 22 percent somewhat supported it.

Contact staff writer Katherine Marks at (760) 740-3529 or kmarks@nctimes.com.