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With his victory in Indiana yesterday, Donald Trump is now, as he claims, the "presumptive" Republican presidential nominee. Although the polls indicate he'll likely lose to Hillary Clinton, the election is half a year away, and a lot can happen in between. The consequences of Trump's becoming president would be momentous for both America and the world. It would change forever the way we think about democracy-and the way the world thinks about America. In fact, his nomination alone will have a deep impact even if he ultimately loses.

A major-party nomination legitimizes a candidate's views as worthy of fair consideration. As a "birther" doubting Barack Obama's citizenship, Trump could be treated as a crank. In the early stages of the primary campaign, his statements about Mexicans and Muslims could be regarded as the wild fulminations of a candidate who would surely be rejected by his party in the end. When his followers attacked protesters at his rallies and Trump himself encouraged and defended their attacks, many people expected that the public would recoil from his thuggishness.

This was not America-at least, so we thought.

But with the nomination in hand, Trump will have the nation's full and even respectful attention for whatever provocations he finds it useful to make. The charge he made this week that Ted Cruz's father was involved in the assassination of John F. Kennedy will not be the last of his reckless accusations. Since a brazen indifference to the truth has worked out well for Trump, we can only expect more of it. The ugliness and degradation he has brought to the Republican primaries he stands ready and able to bring to the general election.

Democracy was already in poor health before this election. Republican obstruction has made it impossible to attend to many of the country's deep and persistent problems; Congress has repeatedly ground to a halt and been unable to act on pressing issues. Special-interest money and influence have intensified cynicism about politics and undermined trust not only in national leaders but also in national institutions. Trump has taken advantage of the resulting disgust with the "system" even though he has been one of the people buying influence, as he proudly acknowledges.

It is fair to say, as many have, that the Republican Party establishment has brought Trump on itself by inflaming the passions of its conservative base, playing to white nationalism, and blocking constructive action on such issues as immigration.

But the GOP has done more than bring Trump down on itself; it has given him legitimacy and put him in reach of the nation's highest office.

The United States has been remarkably fortunate throughout its history. Democracy is not an unerring method for choosing wise leadership, and certainly not all of our leaders have been wise. But the United States has never had a strongman in the Latin American style. We have not had a thug who put constitutional government at risk.

For a century, the democratic countries of the world have been able to count on America's stable leadership. Trump's nomination will shake the confidence that they can continue to count on us. It will also shake the confidence in democracy of many Americans, both Republicans and Democrats, who abhor both the substance and style of his politics. As the election of a black president in 2008 renewed faith in American politics, so Trump's nomination will lead many to doubt whether they can depend on the choices of their fellow citizens. This may turn out to be of the biggest costs of Trump's rise to respectability and the GOP's descent into darkness.