Cindy McCain made an emotional plea Friday in an op-ed in USA TODAY for Americans to honor her husband, Arizona Sen. John McCain, by following his example and fighting for a cause greater than themselves.

"This was the essence of John McCain’s message to our nation," she said. "We must join together, shaped by our deepest values of liberty, equal justice, and respect for the dignity of all people, hardened by unwavering courage and integrity, and step into the arena to fight for a greater cause."

From his military service to his time in Congress, McCain spent his life fighting for his ideals, Cindy said. "With his passing, America inherently understood that this is the kind of leadership we want. That this is the kind of country we want to be."

McCain, 81, died last month from brain cancer and was honored in a series of moving services. Cindy McCain said thousands of people have asked how they best can honor his legacy.

"I’ll tell you how," she said. "Join me in the arena. Fight for a greater cause."

She urged people to support the work of the McCain Institute for International Leadership, where "we will pay his legacy forward." The McCain Institute is based in Washington, D.C., and is run in partnership with Arizona State University. The self-described "do-tank" aims to advance "character-driven global leadership," promote democracy and human rights, keep a strong national defense, and serve "causes greater than one’s self-interest."

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McCain was a Republican throughout his political career, but his identity as a maverick who was willing to put country above party earned him admiration from across the political spectrum.

"So much of our politics, our public life, our public discourse can seem small and mean and petty," former President Barack Obama said at McCain's funeral. "Trafficking in bombast and insult and phony controversies and manufactured outrage. It's politics that pretends to be brave and tough, but in fact is born of fear. John called on us to be bigger than that. He called on us to be better than that."

Former President George W. Bush said: "Wherever John passed throughout the world, people immediately knew there was a leader in their midst. In one epic life was written the courage and greatness of our country."