Franklin Roosevelt wrote a letter to  DemocratS in  2020 

Arguably the most popular president in American history who gave us the GI Bill, the New Deal, Social Security and won WW2 told the democrats what was wrong with their party. It's as fitting today as it was in 1940.

MATTHEW COOKE - NOVEMBER 19, 2019

When FDR ran for his third term he named his Secretary of Agriculture, Henry Wallace, as his VP. Southern democrats thought Wallace was "too liberal." So FDR wrote this letter. 

Franklin D. Roosevelt Letter to the Democratic Convention
July 18, 1940

Members of the Convention:

In the century in which we live, the Democratic Party has received the support of the electorate only when the party, with absolute clarity, has been the champion of progressive and liberal policies and principles of government.

The party has failed consistently when through political trading and chicanery it has fallen into the control of those interests, personal and financial, which think in terms of dollars instead of in terms of human values.

The Republican Party has made its nominations this year at the dictation of those who, we all know, always place money ahead of human progress.

The Democratic Convention, as appears clear from the events of today, is divided on this fundamental issue. Until the Democratic Party through this convention makes overwhelmingly clear its stand in favor of social progress and liberalism, and shakes off all the shackles of control fastened upon it by the forces of conservatism, reaction, and appeasement, it will not continue its march of victory.

It is without question that certain political influences pledged to reaction in domestic affairs and to appeasement in foreign affairs have been busily engaged behind the scenes in the promotion of discord since this Convention convened.

Under these circumstances, I cannot, in all honor, and will not, merely for political expediency, go along with the cheap bargaining and political maneuvering which have brought about party dissension in this convention. It is best not to straddle ideals.

In these days of danger when democracy must be more than vigilant, there can be no connivance with the kind of politics which has internally weakened nations abroad before the enemy has struck from without.

It is best for America to have the fight out here and now.

I wish to give the Democratic Party the opportunity to make its historic decision clearly and without equivocation. The party must go wholly one way or wholly the other. It cannot face in both directions at the same time.

By declining the honor of the nomination for the presidency, I can restore that opportunity to the convention. I so do.

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FDR never had to send the letter. His wife Eleanor Roosevelt gave a rousing speech about unity on the convention floor that won over the delegates. FDR went on to win his third term, defeat the Nazis, grow the middle class, and never stopped fighting big business interests to provide basic economic rights to all Americans. Sound familiar?