Here's the fail-safe test for whether a political party is growing and strengthening or shrinking in size and prospects: Is that party spending its time, energy and effort seeking, recruiting and welcoming converts to its ranks, or is that party instead hunting down heretics within its ranks and, in the name of political purity, banishing them to some outer darkness?

Because American politics is always a matter of addition, not subtraction, the convert-seeking and convert-welcoming party is healthier and almost always has the better prospects of winning the November general election. Republicans understood that well in 1980 when the GOP presidential nominee openly courted and embraced converts, even giving them very own designation as Reagan Democrats. At the 2004 Democratic National Convention, the future Democratic nominee told the nation: "There is not a liberal America and a conservative America. There is the United States of America." Thus did Barack Obama become the first Democrat in 12 presidential elections to win 53 percent of the national Vote.

When former Defense Secretary and Marine Corps Gen. James Mattis overcame his strong personal conviction about keeping the U.S. military out of American politics and vice-versa and stated: "Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try. Instead he tries to divide us. We are witnessing the consequences of three years of this deliberate effort," he captured the nation's attention. Mattis added: "We are witnessing the consequences of three years without mature leadership ... Never did I dream that troops ... would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens" in the service of a "bizarre photo op" of the running-for-reelection president awkwardly holding a Bible in from of Washington's St. John's church.

And while many applauded Mattis' unequivocal indictment, and more than a few lauded former Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey ("America is not a battleground. Our fellow citizens are not the enemy") and Adm. Mike Mullen (Trump "laid bare his disdain for the rights of peaceful protest in this country, (and) gave succor to the leaders of other countries who take comfort in our domestic strife"), immediately, Democratic heretic-hunters were on the offensive. Why did it take Mattis —-as well as Mullen and Dempsey — so long to break with Trump? Didn't they support the ill-fated U.S. wars in Iraq and Afghanistan? Why were they blind — unlike "us"— to Trump's defects of character and intellect?

Winning is not the most important thing in presidential elections. No, winning is the only thing. Nobody understood this better than a fellow named Ronald Reagan, who, in two presidential elections, carried 93 of a possible 100 states. When told of a Republican senator who had disappointed his White House on a key vote and upon whom political retribution was being urged by Reagan's lieutenants, the Gipper had this piece of wisdom: "The person who agrees with you 80 percent of the time is a friend and an ally, not a 20 percent traitor." Democrats in 2020 would be wise to heed that winner's advice.

To find out more about Mark Shields and read his past columns, visit the Creators Syndicate webpage at www.creators.com.