WASHINGTON — A year ago, when Vice President Joe Biden revealed in a television interview that he supported same-sex marriage, such unions were legal in six states.

Tuesday, the Legislature in Biden’s home state, Delaware, voted to become the 11th such state, part of a rapid shift on the issue that is making same-sex marriage the norm in liberal parts of the country. The Delaware Senate approved the marriage bill, 12-9, sending it to Gov. Jack Markell, who has championed the measure.

Delaware’s action, combined with Rhode Island’s passage of a similar law last week, means that same-sex marriage is now legal in most of the Northeast, from Maine through Maryland, with the notable exceptions of Pennsylvania and New Jersey, where Gov. Chris Christie, the state’s Republican chief executive, has blocked a marriage bill passed by the Legislature.

The legislative battles on the issue are now moving to the Midwest, where the Minnesota House is expected to vote on a marriage bill later this week. The outcome there hinges on a few legislators, mostly members of the state’s Democratic-Farmer-Labor Party representing rural parts of the state, who have not yet revealed their positions. Opponents have publicly said, however, they are losing ground.

Gay rights supporters are “hopeful” about the Minnesota outcome, said Evan Wolfson, president of Freedom to Marry, one of the chief advocacy groups on the issue.

The year’s biggest prize for supporters of same-sex marriage would be Illinois, where a legalization bill has passed the state Senate, but faces a more difficult fight in the House.

Supporters say they are closing in on the votes they need, but with the Legislature’s spring session entering its final weeks, they have not yet brought the measure to the House floor. Gov. Pat Quinn has promised to sign the bill if it passes both houses.

The first several states to allow gay and lesbian couples to marry legally did so as a result of court decisions, beginning with a ruling in Massachusetts in November 2003. But a shift accelerated sharply in the last two years, starting with the vote in New York in 2011 that legalized same-sex marriages in that state. Last year, voters in Maryland, Washington and Maine approved referendum measures, marking the first times that the issue had won at the ballot box.

Backing for “marriage equality,” as supporters call it, has now become almost a requirement for the Democratic Party’s elected officials, at least on the national level, and the move toward legalization in Democratic-leaning states seems likely to continue. So far, however, there has been little sign of a shift in more conservative, Republican-leaning states, presaging a divide on the issue that could last for many years.

The Supreme Court heard arguments earlier this year in a case challenging California’s Proposition 8, which barred same-sex marriages in the state. The justices could use that case to require all states to allow same-sex marriages, but when the case was argued, their comments indicated that they are unlikely to do so. A ruling on that case likely will come in June.

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