With gay marriage being argued before the Supreme Court this week, a parade of Senate Democrats has stampeded forth to endorse marriage equality. In a sense, these Senators are hurrying to get in just under the historical wire, as it were, before the court reaches its decision and makes the direction of this debate inescapably clear. The latest is North Carolina’s Kay Hagan, who has now endorsed gay marriage along with other red or purple Senate Democrats who have done the same in the last week, such as Claire McCaskill, Mark Begich, John Tester and Mark Warner.

As best as I can determine, there is only one Democrat in the Senate from a red or swing state right now who voted against the Defense of Marriage Act back in 1996: Sherrod Brown of Ohio. His perspective is particularly interesting, because he voted against DOMA in a state where gay marriage was so unpopular that Republicans were able to use it to turn out voters by referendum eight years later, in the 2004 presidential election.

“When I made that vote, it was politically unpopular — today that position is politically popular,” Brown said in an interview today. “The No votes on DOMA were almost all east coast or west coast.” Brown voted against DOMA as a member of Congress representing a district southwest of Cleveland.

“Casting a controversial vote forces us as elected officials to go home and take a public opinion bath,” Brown said, in a reference to Lincoln’s famous formulation. “Advocating for a position, you can move the public.”

Case in point: gun control. A number of red state Democratic Senators are refusing to say whether they will back expanded background checks, even though by any reasonable measure this is not all that tough a vote, given that the proposal is supported by nine in 10 Americans. (In a hint of movement, Senator Hagan has now indicated she is open to the idea.)

Brown declined to comment directly on his colleagues’ skittishness about background checks, or on the sudden rush among them to support marriage equality, now that the country has shifted dramatically on the issue and the Supreme Court looks on the verge of striking DOMA down. But he noted that the episode carries lessons for Dems who are eying difficult votes in swing or red areas.

“People were very critical after the vote — there was a lot of disagreement and anger about it,” said, Brown, who won reelection by a comfortable margin last year despite running an aggressively populist campaign in the face of tens of millions of dollars in outside cash. “To me, taking a controversial position, if you believe it and you argue it, you can convince enough people that even if they don’t agree with you, they’ll appreciate that you stand for something.”

Take note, red state Dems. It is possible to win arguments over difficult, controversial issues, even when the politics look daunting. And if you try it, you just might find yourselves on the right side of history.

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UPDATE: Post edited for accuracy.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.