As the votes were counted late Tuesday night, it became increasingly apparent that the battle over Measure AA, which would authorize $300 million in bonds for the Midpeninsula Regional Open Space District, might not be decided for quite some time.

The measure, which would use general obligation bonds to fill in gaps in the existing trails system as well as conserve additional habitat for wildlife, was right at the two-thirds percentage mark required for approval with votes still being counted in Santa Clara County.

"It feels like we're very much in the game," said Walter Moore, president of the Peninsula Open Space Trust, the lead sponsor for the "Yes for Open Space" campaign. "It's going to be a long night. We always knew that the two-thirds margin would be very difficult. But voters clearly place a high value on protecting our environment."

This marked the first time that the district, which has preserved nearly 62,000 acres of rural land since 1972, has asked for funding through a bond measure.

Only about 56 percent of its open space currently has public access. The measure, proponents said, would increase that substantially as the funds go toward more quickly completing 25 "priority actions" outlined in its vision plan.

The cost for property owners in the 17 cities in San Mateo, Santa Clara and Santa Cruz counties that make up the district would never exceed $3.18 per $100,000 of assessed value. With the average property in the district costing $700,000, voters would be taxing themselves about $21 annually.

The required two-thirds vote presented a stiff challenge. But the measure also had big money support as the "Yes for Open Space'' campaign raised more than $827,000 in contributions between Jan. 1 and May 17, from organizations such as the Peninsula Open Space Trust and Sempervirens Fund, as well as individuals in the Silicon Valley high tech world.

The San Jose-based Silicon Valley Taxpayers Association opposed the measure, saying that the $300 million plus interest was a bill that future generations will be saddled with paying.

"It's so close that I can't imagine that there will be a final call anytime soon," Moore added.

Follow Mark Emmons at Twitter.com/markedwinemmons.