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The Senate’s passage now gives opponents the official go-ahead to begin a petition effort to force a referendum.

Petitioners would have until June 30 to collect 55,736 valid signatures from registered voters. If successful, the law would be suspended and put on this November’s ballot.

Lawmakers on both sides of the issue have acknowledged that gay marriage will likely go to referendum, especially in the wake of last year’s successful petition effort against the state’s so-called Dream Act.

Opponents gathered 108,000 valid signatures against the law, which would have allowed many college-aged illegal immigrants to pay in-state tuition rates.

Delegate Neil C. Parrott, Washington Republican, helped lead the effort, which owed much of its success to the use of — a website that allowed residents to download and circulate their own copies of the petition.

Mr. Parrott said gay-marriage opponents will likely use the same website this year. He said organizers must first get language in the petition approved by the state Board of Elections and attorney general, but that they hope to begin collecting signatures in as few as two weeks.

Many gay-marriage supporters have bristled at the thought of a referendum, arguing that the bill is a civil rights measure and should thus not be left in the hands of a potentially unwilling public.

Mr. O'Malley said he understands such concerns but has faith that voters will make the right decision. Recent polls show Marylanders to be divided almost evenly on the issue.

“I have a great amount of trust, faith and intelligence of the people of Maryland,” the governor said. “So I don’t fear their judgment.”