And among the most important, I found, was Rubio’s cowardly abandonment, as a US senator from Florida, of a bipartisan immigration reform bill that might have robbed Donald Trump of his most potent issue.

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That legislation came to a head in 2013.

But to understand its origins, we have to go back further in history.

In the early 20th century, America had a highly restrictive quota system aimed at preserving America as white, Anglo-Saxon, and Protestant. The one exception was revealing: Immigrants from countries within the Western Hemisphere were exempt from quotas. Then, as now, America needed Hispanic workers.

Through the dawn of the millennium, business-friendly Republican officeholders strongly supported this flow of inexpensive labor. Rejecting anti-immigrant rhetoric, they praised immigrants for seeking a better life for themselves and their families — personifying the American dream while contributing to our growth and prosperity.

Their support extended to undocumented immigrants — whose children, George H.W. Bush argued in a 1980 presidential debate, should be allowed to attend public school. The Republican hero of that century, Ronald Reagan, opposed building a border wall and supported amnesty for the undocumented. In 1986, Congress passed and Reagan signed an immigration act that tightened border security but granted an estimated 2.7 million such immigrants the opportunity to gain legal status.

In his last speech as president in 1989, Reagan praised immigrants who made America “a nation forever young, forever bursting with energy and new ideas.” For Republicans, hospitality to newcomers from Latin America — legal or not — seemed more or less settled.

Advocating for immigration reform in the early 2000s, President George W. Bush expressed sympathy for undocumented “workers who seek[ing] only to earn a living end up in the shadows of American life.” But Bush’s proposed legislation narrowly failed. Fueled by racial and economic resentment and the fear of Islamic terrorists stemming from 9/11, the Republican base was growing more hostile to nonwhite immigrants.

But if GOP politicians sensed a shift, they did not appreciate just how far to the right the base had moved.

At the beginning of 2013, a bipartisan group of eight senators popularly known as the “Gang of Eight” proposed an immigration reform measure. This group included such Republican eminences as John McCain and Lindsey Graham. Conspicuous among them was Florida Senator Marco Rubio, a young Tea Party darling who, as a Hispanic, was deemed so potentially appealing to minority voters that many Republicans already saw him as a prime presidential candidate for 2016.

The ambitious Rubio desired a legislative achievement to buttress his credentials. In turn, his fellow GOP proponents of the bipartisan immigration reform measure hoped that Rubio’s conservative charisma would help assure its passage. The immigration bill had broad support from progressives as well as conservative intellectuals, economists, think tanks, and influential publications including The Wall Street Journal.

To assure success, the Gang of Eight sought to address concerns across the ideological spectrum. Among the legislative provisions were a major investment in border security and a path to citizenship for undocumented immigrants living in the United States. To demonstrate his commitment to the cause, Rubio made impassioned speeches supporting the bill inside and outside the Senate.

To his apparent surprise, the Republican base — including the Tea Party — erupted in anger. Their principal objection was that the bill constituted “amnesty” for undocumented workers. Their goal was to expel the roughly 11 million “illegal aliens” living in the United States, an expression of their primal fear that white Americans were losing their country to nonwhite intruders.

Fearful of being challenged from the right in GOP primaries, some House Republicans began to back away. So, cravenly, did Rubio.

The political winds might have been picking up, but the Florida senator could have held his ground. He could have harked back to an older Republican politics — and a democratic decency — that had not yet vanished. He could have helped push through landmark legislation, cooling what would only become a hotter fight, while demonstrating his capacity for principled leadership.

Instead, he turned tail. When the reform bill passed in the Senate, Rubio did not appear with his colleagues at their triumphant press conference. Within weeks, Rubio was urging House Republicans to reject the legislation he had long championed. As Frank Sharry, a leading immigration activist, told the Los Angeles Times: “He betrayed our cause in a way that set us back years. … To speak with equal conviction in a matter of months on [opposing] positions, that to me represents a fraudulence and a lack of character.”

The timing of this political cowardice was awful. With a considerable assist from Rubio, his own bill was defeated — foreclosing America’s last chance at comprehensive immigration reform. But his about-face came too late to placate the GOP base, leaving Rubio to serve as a cautionary tale to other Republicans hopeful of resolving our immigration morass. When he ran for president in 2016, irate primary voters doomed his chances. They had found their champion: Donald Trump.