Students celebrate the passage of Proposition A in the DUC.

St. Louis County voters overwhelmingly approved Proposition A, a sales tax measure for Metro, with substantial student support on Tuesday, sending supporters gathered at Washington University into celebration and paving the way for the expansion of transit service in the St. Louis region.

“It’s a blowout,” said Rose Windmiller, the director of state relations and local government affairs at Washington University and a Prop A supporter, after students started a chant of “Metro! Metro! Metro!” in the background. “It feels tremendous.”

The final vote was 62.9 percent “yes” to 37.1 percent “no.” With 84.8 percent of the vote counted by 10 p.m., the “yes” vote was 62.2 percent and the “no” vote was 37.8 percent, meaning a margin that supporters and opponents acknowledged was insurmountable with just 15 percent of precincts having not reported.

More than 100 supporters who were gathered in the Danforth University Center’s Tisch Commons for an election watch party embraced and broke out into cheering as Chancellor Mark Wrighton, Chesterfield Mayor John Nations and Citizens for Modern Transit Executive Director Tom Shrout took the podium just after 10 p.m. The three hailed the proposition’s passage as a victory for the St. Louis region’s residents, students and businesses and not for candidates or parties.

“We win,” Wrighton said to thunderous applause. “St. Louis has made an investment in its future. We will all be rewarded.”

Prop A will raise the St. Louis County sales tax by 0.5 percent and trigger a 0.25 percent tax in the city, raising nearly $80 million a year for future light-rail expansion and for restoring the service that Metro cut on March 30, 2009.

The outcome represented a hard-fought, long-sought victory for supporters, after similar tax proposals failed in 1997 and 2008. And students and administrators, who were elated and exhausted after several days of mobilization efforts, said the measure’s passage means students, faculty and workers at the University will be better connected to the region.

“This is an opportunity for Wash. U. to continue its strong connections to the community, to really connect with St. Louis and to grow as an institution,” said Liz Kramer, an administrative fellow who spearheaded the student-led pro-Prop A campaign.

A small group of opponents gathered at Caldwell’s on the Plaza in Frontenac. St. Louis Tea Party leader Gina Loudon, the wife of former state Sen. John Loudon, R-Chesterfield, said opponents “were tremendously disappointed, but I’m not altogether surprised.” She added that the opposition had already started discussing how to make sure that Metro was accountable to the public, amid their concerns that Metro did not have a concrete plan for the tax money.

Metro CEO Bob Baer pledged accountability, saying the victory was a vote of confidence by the public. “We promise and pledge public accountability, transparency and the provision of the best service we can possibly provide,” Baer said.

With the future of public transit and sales taxes in the region on the line, both supporters and opponents were nervous and optimistic while they waited for results. But as precincts started reporting and the “yes” vote far exceeded the “no” vote, supporters grew more optimistic and opponents realized they faced overwhelming odds.

Tuesday’s turnout was 22 percent, two percentage points higher than what county officials predicted. Kramer said Tuesday afternoon that the Westgate polling place was empty, but other student volunteers reported a steady stream of student voters at polling places on and near campus.

Turnout in other parts of St. Louis County was described as light, especially in West and South counties. In a early positive sign for Prop A supporters, turnout in South County, whose voters likely lean toward “no,” was described as very light by multiple campaign leaders on both sides. But supporters said they needed substantial turnout in North County and Mid County and at Washington University and other schools to have a good chance of passage.

Students and administrators leading pro-Prop A efforts worked feverishly Tuesday to turn out as many students as possible before polls closed at 7 p.m. Meanwhile, the cash-starved local opposition called and e-mailed supporters and tried to gain as much media exposure as possible.

Campus transit advocates’ efforts and the administration’s efforts made up one facet of the broader push by regional Prop A supporters. The Advance St. Louis campaign, though not officially affiliated with the student-run efforts, spent close to $1 million, thanks in part to $75,000 in contributions from the University. Local organizations conducted their own turnout efforts as well.