Initial returns tallied Tuesday night showed the measure was failing 55 percent to 45 percent.
More returns will be posted by late Wednesday afternoon in the mail election, said King County Elections spokeswoman Kim van Ekstrom.
Proposition 1 sought a $60 car-tab fee and a one-tenth-of-a-cent increase in the sales tax. The measure would raise about $130 million a year for 10 years, with 60 percent going to Metro Transit for bus service and 40 percent to pay for roads.
King County said that without the new money, it would be forced to cut 16 percent of its current bus service, or about 550,000 hours, to balance its budget. The agency recently scaled back those figures slightly, after initially saying it would have to cut 17 percent.
Measure supporters said those cuts would hurt working families, seniors and students who rely heavily on buses to get to work, school and services. People who don't use the buses would benefit from money for road fixes and other improvements, and preserving transit would help keep more cars off the roads, supporters said.
Meanwhile, opponents urged voters to reject the measure, saying the agency has problems with "irresponsible spending" and should rein in "excessive operating costs" before asking voters for more money.
If the measure were approved, the sales tax would go up a tenth of a percent to 9.6 percent in Seattle and many cities in the county. The car-tab tax would be set at $60 per vehicle with a $20 rebate for low-income people. The measure would cost the average household about $11 a month.
Proposition 1 has been backed by a number of labor, environmental, civic and community groups, including the League of Women Voters, the Downtown Seattle Association, Sierra Club and mayors of a number of cities in King County.
A leading opposition group, Families for Sustainable Transit, lists the support of many Republican legislative districts, some business groups and others. The Seattle Times editorial board came out against the measure.
County elections officials anticipated a 38 percent turnout in the special election. All ballots must be postmarked by April 22.