As Donald Trump inches ever closer to securing their party’s presidential nomination, Republican leaders are pitifully lamenting: “How did this happen?” The answer is that deliberate immediate, medium and long-term decisions by Republican leaders themselves brought extremists into the party, propelled them into the limelight and imposed the electoral structure that has ceded control to a demagogue.

Mr Trump often recites a ­poem in which a woman is taunted by a snake who poisoned her after she rescued it, telling her “you knew I was a snake” all along. He is trying to castigate immigrants, but his analogy far better describes how his own radical followers seized control of the Republican Party.

In the immediate term, Republican leaders crafted a primary system designed to produce a clear early winner. The intention was to avoid a debilitating internal battle by producing an obvious leader. The system worked all too well. It did indeed yield a decisive early front-runner, but, because of a series of cynical medium and long-term strategic party decisions, it has been hijacked by a dangerous charlatan.

This turn to chauvinism, xenophobia and barely disguised racism is a catastrophe for the long-term prospects for the Republican Party in an increasingly diverse, heterogeneous and tolerant society. Party leaders understand that such a profile will make it almost impossible for Republicans to win the White House and difficult to retain control of the Senate. A Trump nomination is potentially toxic enough to even threaten the Republican majority in the House of Representatives.

This disaster was set up by the Republican leadership in the aftermath of the crushing victory by the Democratic Party, led by Barack Obama, almost eight years ago. President George W Bush left office haunted by the Iraq war and the fiscal meltdown, and a series of notable failures of leadership, particularly the bungled response to Hurricane Katrina. This allowed Mr Obama to sweep into power and, much more alarmingly, for his party to seize control of both houses of Congress.

Americans like divided government, and rarely allow a single party to dominate both houses of Congress and the executive simultaneously. A course correction was almost inevitable. But Republican leaders, panicked at the scale of their defeat, unwisely unleashed a wave of ultra-right wing “know-nothing” populism called the Tea Party. This supposedly grass roots movement was actually well-funded and carefully directed from the top.

The Tea Party phenomenon was promoted to regain control of the House by winning a large number of smaller, local elections where its nativism and chauvinism would resonate. The strategy worked, and Republicans were able to regain control of the House and eventually the Senate. But, once unleashed, getting the genie back in the bottle has proven much more difficult than anticipated.

It’s no surprise that Mr Trump first came to political prominence as a leader of the “birther” movement that promotes suggestions that Mr Obama was not born in the United States. Such rhetoric was a staple of the early Tea Party movement, and brought together the racism, chauvinism, nativism and conspiratorial hysteria that characterises the right-wing “paranoid style” in US politics.

The most disturbing qualities of Trump rallies, including the rage and violence that courses through them, was previously expressed at Tea Party events. The Trump campaign is the obvious, logical conclusion of the Tea Party movement, and for Republican leaders a Frankenstein’s monster now turned against its own creator.

But the deepest seeds of the Trump phenomenon were sown by the Republican Party many decades ago. As Rick Perlstein describes in a brilliant series of books beginning with Nixonland, in the mid- to late-1960s Richard Nixon and his successors, most notably Ronald Reagan, redrew the political map and divided the US politically as it is to this day.

Nixon created a new Republican governing majority by combining the rising right-wing populism of the Sun Belt states, including the nascent religious right, with the old segregationist, racist constituency in the South that left the Democratic Party after the civil rights movement. Nixon and Reagan’s new constituency brought into the Republican tent the racists, chauvinists, demagogues, paranoiacs, conspiracy theorists and hysterics who have found their ultimate expression, and victory, in the Trump campaign. Buzzwords about “law and order” and “states’ rights” blew the dog whistle that summoned extremists into Republican ranks.

These racists, reactionaries and religious fanatics were not supposed to take over, of course. They were supposed to rage and fulminate impotently about minorities, immigration, socialism, Jews and Muslims, homosexuals, abortion, evolution, climate change, one-world government and so forth, and dutifully vote Republican. But not set the agenda.

It’s no mystery how extremists captured the Republican Party. Nixon invited them in. Reagan kept them there. The tea party catapulted them into national political prominence. The primary system provided the practical mechanism to seize control. For all his bluster, Mr Trump is merely their expression and their vehicle.