Hopes dimmed over the weekend for passing a measure to double Alameda County's transportation sales tax to a full one cent, but the counting continues.

The Measure B1 sales tax increase was trailing Monday with 65.79 percent of the vote in support -- short of the 66.666 percent required for approval. That includes some 93,779 more votes counted over the weekend, county election officials reported Monday.

There are still tens of thousands of provisional ballots to be counted on the measure to boost spending for roads, freeways and public transit, election officials added.

While the percentage favoring the measure climbed slightly in the latest tally, transit officials acknowledge they face an uphill struggle to reach the two-thirds total to pass.

"You can imagine this dampened spirits," said Tess Lengyel, deputy director for the Alameda County Transportation Commission, the agency that proposed the tax increase. "It's still not over, though. We are going to continue to watch the results until the counting is finished."

As of early Monday afternoon, the measure was supported by 318,089 voters -- more than 4,200 votes short of two-thirds.

The measure would boost spending for roads, freeways and public transit, including providing $400 million toward a proposed BART rail extension to Livermore.

The measure was supported by city and county leaders, and many business and environmental groups as providing relief for congestion.

Opponents said the tax was too high and they also objected that the ballot measure would lock in the tax as permanent, unlike previous sales tax measures that expired after a set period unless renewed by voters.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff