I was first elected to Congress in 1964. That was the year Lyndon Johnson won a full term as president in a landslide. If ever a president had a popular mandate to pursue his goals, it was LBJ in the few years that followed that election.

Yet one of my strongest memories of him is not of a president reveling in partisan supremacy, but of his cautioning against it. Johnson used to love meeting with freshman members of Congress, and after taking office we Democrats who’d been elected along with him had every expectation that he would allow us to bask at the expense of our Republican colleagues. He didn’t. “I’m an American first,” he told us. “And I’m a Democrat second.”

It was a bracing affirmation of a quality essential to national leadership — a firm conviction that the good of the country comes first, even if it runs counter to the interests of one’s political party. I can’t help thinking of it today, in an era when deep, seemingly unbridgeable differences divide Democrats and Republicans, and when these divisions are being stoked by the current presidential campaign.

It has been apparent almost since the beginning that our nation’s welfare rides on how well political leaders balance the needs of the country against their partisan goals. In 1796, preparing to step down from the presidency, George Washington devoted much of his Farewell Address to this question, and to the destructiveness of what he called “the fury of party spirit.”

Surveying with alarm the regional discord and the growing hostility between Federalists and the Republicans that took hold in the final years of his second term, he set out to warn Americans that the very permanency of the Union depended on “a government for the whole.”

Other national leaders understood the sentiment. Patrick Henry’s famous statement, “United we stand, divided we fall” was followed by these words: “Let us not split into factions which must destroy that union upon which our existence hangs.”  “We are all Republicans, we are all Federalists,” Thomas Jefferson said in his first inaugural address.

Each of these leaders signaled a bedrock belief in the importance of working together to bridge differences and find common ground because the nation’s welfare demanded it, regardless of the dictates of a party’s extremes.

Now, I’m not urging that we be naive. We’re not going to abolish parties, and we shouldn’t. They help us organize our political choices, define and advocate issues, and make sense of elections.

But if we’re not careful, they can be carried to such an extreme that they divide government, when what we need is unity of government. We need it in foreign affairs, where the more united we are as a nation, the stronger we are. And we need it in domestic policy, where excessive partisanship agitates the people and creates animosities among them. It leads to distrust within Congress, mistrust of Washington, weaker administration of government, and an inability to resolve the problems that press against our future. If you doubt any of this, just look around.

It is extraordinarily difficult to create a government that works together for the common good. One reason most presidents end up talking about the unity of the country and of government is because they, more than most of us, can see the centrifugal forces of region, ethnicity, religion, and ideology at work. They know that there is no magic formula for balancing them all.

But in this era of unforgiving partisanship, it is too easy to forget the importance of trying — and of working hard not to fan the flames of divisiveness. It is crucial to avoid painting the other side as un-American or eager to betray the national interest, just as it is to recognize that we have more in common than we have differences.

Our differences are important; they are part of who we are as a nation. But if we want to overcome our challenges and preserve our greatness, unity is indispensable. The great work of our democracy, as it has been for over 200 years, is learning how to reconcile the two.

Lee Hamilton is Director of the Center on Congress at Indiana University. He was a member of the U.S. House of Representatives for 34 years.