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In Maine, a bill to legalize same-sex marriage passed the Senate on Thursday by a vote of 21-14 and now goes to the House, where a vote is expected Tuesday. Most observers agree that House passage is likely.

“We’re very hopeful for the House vote,” said Mr. Wu. “We certainly have the momentum after that amazing Senate vote.”

Like Mr. Lynch, Mr. Baldacci had opposed gay marriage in favor of civil unions, but he issued a statement when legislation was introduced in January saying that he would keep an open mind during the debate.

“I’m not prepared to say I support gay marriage today, but I will consider what I hear as the Legislature works to find the best way to address discrimination,” Mr. Baldacci said.

Mr. Baldacci doesn’t face the pressure of a 2010 campaign because of term limits. Opponents of same-sex marriage are preparing for the worst, announcing last week that if he does sign the bill, they will attempt to block it with a veto referendum, known as a “people’s veto.”

Placing such a measure on the ballot is relatively easy in Maine, requiring about 55,000 signatures, and the effort is often successful. A veto referendum to overturn a tax on soft drinks passed in November.