2015 is the year that Donald Trump went from joke candidate to national poll leader, humiliating Jeb Bush and roiling the entire GOP primary. He's now fallen behind Ben Carson in some polls but is still rising well above mainstream candidates while also posing a more profound problem to them than Carson's relatively straightforward economic appeal. To understand what's happening in politics right now, you have to understand Trump.
Your sister-in-law says:
"Trump is the only one who's speaking the truth about immigration."
What's true here is that both Bush and Marco Rubio have urged various forms of leniency to millions of people who immigrated to the United States illegally but have not otherwise broken the law. This is a common view among Democrats, among the GOP donor class, and with many high-level Republican Party strategists, but it's anathema to the Republican rank and file. Trump, meanwhile, has been unusually vocal about his hostility to unauthorized immigrants and has outlined an extremely harsh plan to kick them out of the country.
Another relevant fact is that on net, illegal immigration to the United States has essentially halted in recent years, so there's no particularly clear need for tough new policies to stop people from coming.
Your niece says:
"Trump can't possibly win. The GOP always goes establishment."
There is unquestionably an influential school of thought in political science that says that despite the appearance of open primaries, party elites ultimately control presidential nominations. This idea, long derided by journalists, has become prominent in the media this cycle just as the Republican primary seems to be putting it to the test. Your niece is right that the current polling leads for Carson and Trump shouldn't be taken at face value (remember Herman Cain?).
But the evidence behind the "party decides" theory turns out to be rather weak. Especially in recent elections, party elites have struggled to coordinate behind a single leader — something that is evident in this year's governor-laden GOP field. And in other cases where the outsider lost in the end — think Howard Dean in 2004 — the actual mechanism seemed to involve more work by the voters of Iowa and less work by party elites than establishment-oriented theories show. In other words, while the odds of a nonprofessional with little elite support somehow screwing things up seem reasonably good, you shouldn't count Trump out.
Last but by no means least, it's worth considering the money factor. We've never seen a self-financed candidate contend in a serious way in a presidential primary, which means that conventional wisdom driven by past elections doesn't necessarily apply.
Your father-in-law says:
"Only in America!"
Actually, Trump's combination of anti-immigrant politics, cultural traditionalism, and support for retirement security programs like Medicare is a fairly common cluster of policy positions among new political parties of the populist right in Europe. From the National Front in France to the True Finns, Sweden Democrats, Danish People's Party, and the UK Independence Party, Trumpism is a global phenomenon.
The difference is that the United States has an election system that encourages two-party politics and a presidential nominating system that is relatively friendly to outsider figures. That creates space for a Trump-like figure to emerge inside one of the two established political parties rather than as a new party on the fringe.
Your cousin says:
"The far-right surge Trump represents is terrifying."
Feel free to be terrified if you want, but there's actually little evidence that the Trump phenomenon is "far right," per se. As David W. Brady and Douglas Rivers point out, the actual demographics of Trump supporters do not suggest a right-wing fringe movement. Sixty-five percent of Trump supporters describe themselves as "conservative," which is about what you would expect for a Republican. But among the remaining 35 percent, self-described "moderates" outnumber the self-described "very conservative" element.
"Less than a third of his supporters say they are involved with the Tea Party movement," Brady and Rivers write. "Their views put them on the right side of the American electorate, but they cover the Republican mainstream."
Indeed, while Trump's views on immigration are way out there on the ideological fringe, Trump has also staked out a pro-Medicare position that's to the left of Jeb Bush, and his proposed tax cut is considerably smaller than Marco Rubio's.