(AP Photo/Jacquelyn Martin)   The two major political parties of the United States are imperfect vehicles, to be sure. But when they take stands on fundamental issues, these powerful institutions provide a measure of where not just the politics but the governance of the nation, the states and communities across the country may head if members of that party come to power.

So it’s a big deal when parties take new positions on big issues. When political parties break with the status quo, when they officially move beyond partisan and ideological comfort zones, that’s a moment where the political dynamic is shifting.

It was a big deal in 1948 when—after Minneapolis Mayor Hubert Humphrey told the Democratic National Convention in Philadelphia that “the time has arrived in America for the Democratic Party to get out of the shadow of states’ rights and to walk forthrightly into the bright sunshine of human rights”—the Democrats put a civil rights plank in the platform.

It was a big deal in 1976 when—under pressure from delegates aligned with a rapidly developing religious right movement —a Republican Party that had historically been sympathetic to reproductive rights shifted to an ardent anti-choice position.

It is a big deal now that Democrats are preparing to amend their platform to endorse marriage equality. To be sure, President Obama and Vice President Biden have already begun talking about their support for the right of members of the LGBT community to marry their partners. But that talk becomes far more grounded in reality when the Democratic Party declares for marriage equality and against the Defense of Marriage Act that was signed into law by a Democratic president (Bill Clinton).

On Sunday, in Minneapolis, a Democratic platform drafting committee took the first critical step toward amending the platform on this issue. “I was part of a unanimous decision to include it,” explained Congressman Barney Frank, D-Massachusetts. “There was a unanimous decision in the drafting committee to include it in the platform, which I supported, but everybody was for it.”

That bodes well for approval of the amendment by the full platform committee and by the convention in Charlotte.

“The Democratic Party has a noble history of fighting for the human and civil rights of all Americans,” explained Mark Solomon, national campaign director of the Freedom to Marry movement, who testified before the drafting committee. “We are proud that the Committee is including language that will ensure the Party is leading the way forward in supporting marriage for loving and committed same-sex couples and their families.

Solomon is right to be pleased, as are the representatives of the American Civil Liberties Union, the Stonewall Democrats, the Servicemembers Legal Defense Network and the Human Rights Campaign who pressed the drafting committee to act.

Party platforms do not always define campaigns. Former Senate majority leader Bob Dole famously declared as the 1996 Republican presidential nominee that he had not read that year’s GOP platform.

But on the arc of history platforms are much more than mere pieces of paper. They are outlines of where a party will go if it comes to power. And history has confirmed, again and again, that when major parties move on a major issue, the repercussions go far beyond caucuses and conventions. They focus debates, accelerate activism and ultimately define the direction of the local, state and national legislating—for the better and sometimes for the worse.

The embrace of marriage equality by the DNC’s platform drafting committee should be seen in that context. It is a big deal, and when the Charlotte convention approves the amendment of the platform, the delegates will place the Democratic Party on the right side of history.