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Democrats are laser-focused on winning the 2020 presidential election, but what remains less obvious is the vision that will unify them and appeal to others in November and beyond. Even as they debate often profound policy disagreements, a successful Democratic candidate must inspire Americans with a clearer, bolder vision than President Trump has offered for what America can be in the 21st century. He or she must understand that policy matters, but purpose matters more.

For those seeking inspiration for such an approach, Lyndon Johnson's Great Society offers a compelling case study of what the federal government can achieve and how grassroots activists can help move Washington toward a better place if animated by a clear purpose.

When Johnson delivered his Great Society speech on May 22, 1964, at the University of Michigan, he called Americans to a higher purpose. In the midst of social convulsions and divisions unseen in modern America, Johnson offered a vision of American freedom born of our bedrock values -- tolerance, pluralism and the rule of law -- and he challenged Americans to extend the benefits and responsibilities of those values to everyone, expanding our notion of American citizenship.

Johnson's argument was simple: For America to fulfill its purpose, it needed to expand its definition of freedom to include all Americans, and in so doing build a country where actual economic opportunity might, for the first time, accompany that freedom.

The policy outcomes of Johnson's clarion call remain unprecedented. The Great Society guaranteed health care coverage to the elderly and the medically indigent, ended legally sanctioned segregation, and made it possible for millions of younger Americans to attend colleges and universities, often for the first time in their families' history.

While many conservatives continue to argue the Great Society constituted a massive failure, the programs created a basic floor of rights and benefits that most Americans now consider part of the fundamental fabric of our nation.

In addition to Medicare, Medicaid, civil rights legislation and the Higher Education Act, in a relatively short span of time Congress also passed federal aid for elementary education, food stamps, Head Start, immigration reform that ended the quota system put into place in 1924, arts funding, a war on poverty and more.

While many conservatives continue to argue the Great Society constituted a massive failure, the programs created a basic floor of rights and benefits that most Americans now consider part of the fundamental fabric of our nation. Even as detractors raged against the changes taking hold, those on the left and right built on Johnson's foundation. Richard Nixon's Environmental Protection Agency was the fruit of Johnson's environmental protection legislation. Tea Party activists holding up signs that read "Keep Your Government Hands Off My Medicare" as they opposed President Obama's Affordable Care Act proposal was a significant expression of acceptance on the right of the basic premise of the Great Society.

Some have argued that the Great Society was possible only because Johnson had a Democratic congressional majority. While recreating those kinds of majorities will be extremely difficult in 2020, Democrats can look to another crucial element of why the Great Society happened.

By Johnson's own admission, the achievements of that era were fueled by a movement of women, men and children who aspired to be fully included in the life of the country and rallied with a president whose purpose was their own. Johnson was right. It was a bottom-up movement that gave LBJ a window to do big things in the face of entrenched and formidable adversaries.

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