It is heartening to see some important Republican figures come out against Donald Trump. But it’s worth noting that many embraced him when he proposed a Muslim ban, tried to extort Ukraine’s president, was impeached and tried to overturn an election. His real sin, in their eyes, is that he is losing popularity.

Were Trump to have a revival of his fortunes, many would jump back on his bandwagon. Sen. Ted Cruz (R-Tex.) has already reserved a spot there, promising that if Trump were to get the nomination he would “enthusiastically support him.”

How did we get here? There are, of course, many reasons. But the central facilitating factor is surely the way that U.S. politics has, over the past few decades, increasingly empowered the extremes of political parties at the expense of the mainstream.

In countries where democratic institutions are weaker — such as Hungary, Turkey and (alas) the United States — demagogues change parties rather than the other way around. To fend off the threat, Republican leaders must act to purge their party (and country) of extremism.