One of the great truths about Campaign 2016 is that voters are presented with a presidential candidate named Bernie Sanders who is one of the greatest truth tellers in the modern history of American politics.

Another of the great truths about Campaign 2016 is the Democrats are presented with a frontrunner candidate named Hillary Clinton who, when asked to promise that she will never lie to the American people, gave an answer other than an unqualified “yes.”

Mr. Sanders is the Davy Crockett of American politics. Here is what I mean. The famous frontiersman was also a congressman before he lost his life at the Alamo. He represented a district in Tennessee. While President Andrew Jackson, also of Tennessee, is remembered as a populist it was Congressman Crockett, who battled him in support of low income citizens and American Indians, who was the real populist.

In the latest remake of the film about the Alamo there is a scene where Colonel Travis has learned that the reinforcements would not arrive and the Texans who remained at the Alamo were doomed. Colonel Travis told Mr. Crockett he did not know what to tell the Texans who remained at the fort.

Mr. Crockett told him that when he was in Washington his colleagues made good sport of him. They told him how to dress, what fork to use in polite company, and concluded “I was never afraid to stretch things a bit but I never learned to lie.”

At a time when the leading Republican candidates are calling each other chronic liars and bad Christians, Democrats might reflect on the challenge that lies ahead between now and election day in November.

Ms. Clinton tells only part of this truth—but cannot tell the whole truth—because she wants the credit but she also wants the money.

My thesis about contemporary political history has long been that we live in age of vast social injustice, income inequality and vast financial and political corruptions that have created enormous public outrage against the establishments of all kinds that not only create but prosper from these great wrongs.

In the presidential campaign there is one and only one candidate who has told the unvarnished truth about explaining, challenging and correcting the injustices that cause such rampant inequality throughout American society and that candidate is Mr. Sanders.

It is no coincidence that the great truth-teller about the great issue of the election is a candidate who does not take money from the super-wealthy and the special interests. Nor is it a coincidence that his opponent, who borrows liberally from his words but then attacks—not always honestly—his deeds, has taken a lot of money from the super-wealthy and special interests.

The big lie in Washington today is from politicians who say that they are not beholden to those who give them money. The big truth Mr. Sanders tells, which Ms. Clinton tries to tell to the best of her ability, is that it is this hammerlock of big money that corrupts every issue that challenges America. Mr. Sanders tells the whole truth because he does not want or take the dirty money and funds his campaign from huge numbers of small donors whose only reason for supporting him is they share his dreams for America. Ms. Clinton tells only part of this truth—but cannot tell the whole truth—because she wants the credit but she also wants the money.

Mr. Sanders call for a free public college education financed by a transaction tax on Wall Street speculators. He calls for a single payer healthcare system, which every democratic nation in the world has a form of. He calls for breaking up big banks, a position recently endorsed even by a member of the Federal Reserve Board.

Ms. Clinton criticizes the various issues Mr. Sanders raises, which is fair enough though I agree with Mr. Sanders and disagree with Ms. Clinton on these matters. But then she goes further. After criticizing Mr. Sanders on one issue after another Ms. Clinton then says he is a one issue candidate.

This charge by Ms. Clinton that Mr. Sanders is a one issue candidate is, obviously, a lie Ms. Clinton should stop telling.

The truth about politicians in Washington, which I tell you as someone who has worked for some of the most respected Democratic leaders in the House and Senate, is what Davy Crockett said to Colonel Travis in that film about the Alamo.

Most politicians in Washington hold so many fundraisers, accept so much money, have so many closed-door meetings with lobbyists who hand them checks in return for legislation, hire so many flacks to put words in their mouths, are surrounded by so many consultants and courtiers whose dream is not to change America but to make a lot of money shuffling through the revolving door to cash in with special interests that day after day, year after year, decade after decade they lose sight of the difference between the truth and the lie.

To his enormous and timeless credit Mr. Sanders never learned to lie and that is what makes him such an extraordinary treasure in the public life of the nation.

Many of Ms. Clinton’s supporters actually support single payer healthcare, for example, but because of their support for her they feel obligated to publicly state the opposite of what they believe when they criticize Mr. Sanders.

There is not one supporter of Mr. Sanders who feels obligated to say things he or she does not believe because of their support for him.

As the campaign continues Ms. Clinton would be well advised to spend her time describing her vision and plans for America without the unfounded attacks against Mr. Sanders. She has much to offer the nation and if she is nominated, a huge turnout of his supporters will be necessary for her to be elected in November. That prospect is not enhanced when she stretches the truth a bit too far.

Mr. Sanders will continue his campaign for equality and justice, which is good for Democrats and good for America. He has already expanded his support from young people who to their credit support him devoutly to working class white voters and an increasing number of Hispanics. If he confounds the odds and receives the Democratic nomination the polls now show he would defeat his Republican opponents and runs stronger against them than does Ms. Clinton.

Why does Mr. Sanders run so strongly against Republicans in general election match-ups?

The answer is simple. Like the Davy Crockett character in that film he never learned to lie, and that is treasure that all of the money on earth can never buy.