Gay marriage advocate Beth Robinson, center, holds back tears following the passage of a gay marriage bill in Montpelier, Vt., Tuesday, April 7, 2009. Vermont has become the fourth state to legalize gay marriage. The state legislature voted Tuesday to override Gov. Jim Douglas' veto of a bill allowing gays and lesbians to marry. The vote was 23-5 to override in the state Senate and 100-49 to override in the House. Under Vermont law, two-thirds of each chamber had to vote for override. At left is Sherry Corbin and at right is Susan Murray.

Montpelier, Vt. ? Vermont, which invented civil unions, on Tuesday became a pioneer again as the first state to legalize gay marriage through a legislature’s vote, suggesting growing popular acceptance of the idea.

The House barely achieved the votes necessary to override Gov. Jim Douglas’ veto of a bill that will allow gays and lesbians to marry beginning September 1. Four states now have same-sex marriage laws and other states soon could follow suit.

Bills to allow same-sex marriage are currently before lawmakers in New Hampshire, Maine, New York and New Jersey. The three other states that currently allow same-sex marriage — Connecticut, Massachusetts and Iowa — each moved to do so through the courts, not legislatures.

“For a popularly elected legislature to make this decision is a much more democratic process” because lawmakers have to answer to the voters every other November, said Eric Davis, a retired Middlebury College political science professor.

Courts typically deal with arcane points of constitutional law. While legislatures debate some of the same principles, the process may become much more personal. In Vermont, some of the most gripping debate came when gay and lesbian lawmakers took to the House floor last Thursday and told their own personal love stories.

Getting gay marriage approved in a political, rather than purely legal, forum is a big step, said Boston University law professor Linda McClain, an expert on family law and policy. “What may give courage to other legislatures is that this legislature managed to do it,” she said.

She added that using the civil rights language of equality — the measure in Vermont was dubbed the marriage equality bill — could help make gay marriage more acceptable elsewhere.

Opponents said they, too, believe activists will be emboldened in other states. The action comes just days after the Iowa Supreme court ruled that not permitting gay marriage there was unconstitutional.

“To the millions of Americans who care about marriage, we say get ready: President Obama and Democrats will use Vermont as an excuse to overturn the bipartisan federal Defense of Marriage Act,” said Brian Brown, executive director of the National Organization for Marriage, which waged a radio campaign against the measure. “The next step is to ask the Supreme Court to impose gay marriage on all 50 states.”

The Defense of Marriage Act, signed into law by President Bill Clinton in 1996, defines marriage as a legal union between a man and a woman, and provides that states need not recognize the marriage of a same sex couple from another state.

To date, the same-sex marriage movement’s main gains have been in New England, which some attribute to Yankee liberalism and the gradual acceptance of gay relationships after Vermont’s groundbreaking civil unions law took effect in 2000.

Douglas had announced his intent to veto the gay marriage bill two weeks ago, saying he believed marriage should be limited to a man and a woman and calling the issue a distraction during a time when economic and budget issues were more important.