(Montpelier, Vermont) LGBT civil rights groups in Vermont are mounting a massive campaign to secure a veto-proof House vote on gay marriage.

The House is expected to vote later this week on a marriage equality bill.  The measure passed the Senate last week with a massive 26-4 vote. It is expected to pass the House with a much narrower majority.

Following Senate passage of the bill, Gov. James Douglas (R) announced he would veto it.

Since then same-sex, marriage advocates have been stepping up the campaign to win enough House votes to override a veto.

On Friday, hundreds turned out at the Statehouse in a show of support.  They buttonholed lawmakers and are using a telephone campaign to try and sway opponents and undecided representatives.

“No one said this would be easy,” Senate President Pro Tem Peter Shumlin, D-Windham, told the rally. “But we are more determined than ever to overcome his veto.”

Rep. David Zuckerman said the bill currently has more than 75 strong supporters in the House, but it would take 101 votes to overcome a veto.

Gays have been shadowing Douglas staging noisy protests. So far, Douglas has refused to budge. A state GOP spokesperson said that polling is showing Vermont voters overwhelmingly opposed to same-sex marriage.

Vermont was the first state in the country to legalize civil unions in 2000. Since then, LGBT groups have criticized the law for creating a “two tiered” system – marriage for opposite-sex couples and civil unions for gays.

Under the marriage bil, the civil unions law would allow marriage of same-sex partners beginning Sept. 1. Civil unions, which confer some rights similar to marriage, would still be recognized but no longer granted after Sept. 1.

Supporters cast the debate as a civil rights issue, saying a civil unions law enacted by the state in 2000 has fallen short of the equality it promised same-sex couples. Its appeal has declined, too: In 2001, the state granted 1,876 civil unions, compared with only 262 last year.

An impact study released earlier this month suggests there is a link between the economy and gay marriage.

The study, by the Williams Institute at UCLA, found that approval of gay marriage in Vermont could generate $31 million in new spending and $3.3 million in state taxes over three years.

Last November, Gay & Lesbian Advocates & Defenders, the Boston-based LGBT rights group that brought the successful legal challenges leading to same-sex marriage in Massachusetts and Connecticut launched the “Six by Twelve” campaign to legalize gay marriage throughout all six New England states by 2012.

Opponents say gay marriage would undermine traditional male-female marriage, rendering men and women interchangeable and destroying the connection between children and marriage. They want the question put to voters in a referendum.