Here are the four most significant characteristics the two men share:
1. Both are products of partisan politics.
In 1950, the little-known senator, searching for an issue that would grab the public's attention, declared that he had a "list" of names of communists working in government. Though few people paid him much notice at first, he repeated, expanded, and varied his charges in succeeding speeches. Although he never uncovered a single communist, his antics produced front-page news across the country. His face appeared on the cover of Time and Newsweek, and Republican candidates begged him to make appearances on their behalf.
Republican leaders of the day aided and abetted McCarthy's rise to prominence. Embittered by their stunning loss in the 1948 election, Republicans saw McCarthy as the ideal way to undermine support for President Harry Truman, and guarantee victory in 1952. So long as McCarthy brandished his anti-Communist club against Democrats, respectable Republicans were willing to overlook his provocative and offensive tactics.
All that changed once the Republicans gained the White House in 1952, for the first time in 20 years, with the election of Dwight Eisenhower. Now, McCarthy's perpetual investigations proved an embarrassment to the party and, many of the same people who had publicly supported him a few months earlier, began to turn against him.
In December 1954, following the disastrous televised Army-McCarthy hearings, Republicans joined Democrats in voting to "condemn" McCarthy for bringing the Congress into disrepute.
The Republican establishment has responded to Trump the same way. Republicans were convinced they should have won against President Barack Obama in 2012. They emerged from that defeat determined to regain the White House in 2016, by any means necessary.
While Republicans, like everyone else, underestimated Trump's appeal, they also saw him as a useful vehicle for undermining support for the Democratic Party. That meant ignoring Trump's incoherent, impractical, and inconsistent policy proposals, and remaining largely mute when the billionaire called immigrants "criminals" and "rapists." As long as Trump's reckless charges and baseless attacks appeared to be helping the party by bringing in new voters, Republican were willing to look the other way.
After Trump scored surprising victories in early primary states, and emerged as a viable nominee, Republicans turned against him. They now saw him as a liability who was hurting the Republican brand. Party elders like Mitt Romney, who had remained silent for months, suddenly found Trump's rhetoric distasteful and his style unpresidential. Trumps victories in Mississippi and Michigan suggest the establishment attacks are too little, too late.
2. They brilliantly manipulate the media.
No matter how reckless and irresponsible the candidate's accusations, the media -- newspapers back then, television and digital outlets today -- have provided a platform because McCarthy sold papers just as Trump boosts ratings.
The rules of objective journalism dictated that the press cover McCarthy's outrageous indictments, even if reporters knew they were not true. McCarthy never produced a shred of evidence to support any of his accusations, but his tactics gained him the publicity he needed, and thus fueled his attacks.
Trump plays a similar game today, using controversy and shocking comments to manipulate the media agenda. His attacks on immigrants, war heroes, and women are purposefully designed to generate controversy and keep him at the center of attention. Trump has also used the snarky language of Twitter to set the political agenda, and control the daily news cycle. The networks and cable news programs, desperate for the ratings spike that he provides, have even allowed him to call in his interviews. Like McCarthy, Trump traffics in fear and bigotry, but because he makes money for the major news outlets, they continue to give him a disproportionate amount of air time.
3. They preach phony populism.
Populism has a rich history in American politics, but it has always been a double-edged sword. In the hands of reform-minded leaders it has been used to mobilize majorities and, usually, to expand government power. But in the hands of demagogues it has been used as a way of gaining power by demonizing opponents.
McCarthy became the most talented demagogue of recent time by denouncing liberal elites who he claimed had coddled communists. He called Truman's Secretary of State Dean Acheson a "pompous diplomat in striped pants." He denounced the "egg-sucking phony liberals" who defended "communists and queers."
Trump has chosen to demonize immigrants. He claimed, falsely, that an "Arab community" in New Jersey cheered when the Twin Towers fell on September 11th. He stated that the U.S. should ban Muslim immigrants and keep a database of Muslims already in the United States. He blames violent crime in the United States on African-American and Hispanics, and uses phony crime statistics supplied by a white supremacist group as proof.
4. They tap into real anxieties but offer no solutions.
McCarthy offered simple answers to the complex questions of the Cold War. He reassured a worried nation that a string of foreign policy setbacks resulted from the traitorous actions of a few individuals, not from a flawed view of the world or the strengths of communist opponents. China turned communist, McCarthy explained, because "traitors" in the State Department had sold out American interests, not because of the internal weakness of the Chinese Nationalist regime that the U.S. supported. The Soviets developed the atomic bomb, he said, because spies had sold them America's secrets, not because they had talented scientists capable of developing their own bomb.
Trump taps into the fears of a white working-class struggling with the consequences of globalization, a dramatic shift of wealth to the very rich, and diminished opportunities. Their fears about the future are real (and justified). The possible solutions are complex. What does Trump offer? All the problems facing the nation can be solved, he suggests, by tossing out the "very, very stupid people" running the government, by building a wall on the Mexican border, and by banning Muslims from entering the country.
The paradox of demagogues like Joseph McCarthy and Donald Trump is that they attempt to erode individual liberty in the name of freedom and use legitimate worries to undermine support for the very values they claim to be upholding.