Donald Trump is the Republican Party nominee

So Donald Trump is to be the Republican Party nominee.

Establishment Republicans need to take a deep breath: The US system of checks and balances was designed to preclude the kind of tyranny some are warning could accompany a Trump presidency.

There will not be guillotines in the streets of Washington. Nonetheless, establishment Republicans are right to be shocked: Until recently, ours has been a strikingly successful political party.

The Republicans' governing wing holds majorities in the US Senate, House of Representatives, state legislatures, and governorships.

Under President Barack Obama, Democrats have lost 13 seats in the Senate, 69 seats in the House, over 900 seats in state legislatures, and 12 governorships. It looks like their losing streak may be ending.

Republicans need to prepare for a landslide defeat at the hands of a candidate who has alienated a substantial majority of the country.Republican elites need to acknowledge that significant numbers of voters in our own party support a candidate who does not subscribe to orthodoxy on foreign policy, trade, immigration, and many other matters.

But we also need to prepare for the possibility that, just was we were all wrong about Trump's prospects of winning the Republican nomination, we might also be wrong about his strength against Hillary Clinton.

There is a scenario in which Trump could channel the anger not only of his own voters, but of those who have supported Bernie Sanders, in ways that could actually produce victory in November. This is not a reason to vote for him, but it is a warning.

In the meantime, some hard questions:

1 Sixty per cent of Republicans who voted in the 2016 primaries did not vote for Donald Trump. Where do those voters go?

Some will surely reconcile themselves to their party's nominee. But others will vote for their country over their party by supporting Clinton. Still others will consider supporting a third-party candidate. More will stay home. The fact that a supermajority of Republican primary voters did not support their party's nominee is unprecedented in modern times - and leaves their votes up for grabs.

2 Will Trump be more effective at mobilising his own supporters or those of his opponent?

His campaign foresees an energised base of many millions of voters, including blue-collar Democrats and others outside traditional Republican constituencies, turning out in a tsunami of support for the Republican ticket in November. But perhaps more plausible is the idea that his candidacy will mobilise Democrats, independents, and moderate Republicans to vote against rather than for him.

3 Will having Trump at the top of the ticket help or hurt the Republican majority in Congress?

If he is able to maximise voter turnout, his supporters could vote down the ballot for more orthodox Republicans who will keep the party's governing wing in control of Congress. But if instead he is more effective at rallying his opponents, they could swing the House and Senate to Democratic control. There is even a third possibility: that some Democrats and independents who are among the 60 per cent of Americans who distrust Clinton but will grudgingly support her split their votes to enable a Republican Congress to oversee her administration.

4 Is Trump a gift to the Clinton campaign or a potentially mortal threat to her establishmentarian platform?

She is running to succeed a weak incumbent of her own party whose approval ratings hover below 50 per cent, in a time of economic uncertainty. She is running as a policy wonk rather than a populist, as a leading member of the political aristocracy, and as an enlightened internationalist. Her campaign managers have surely noticed that Americans are not voting for these particular candidate qualities in this cycle.

5 Will a credible third-party candidate emerge to contest both Trump and Clinton?

Mitt Romney would have been a fine president. He actually won a majority of independent voters when he ran in 2012, but lost due to very strong Democratic turnout. Could he again win independents, as well as the majority of Republicans who at present do not support Trump, with disgruntled Democrats (including Sanders voters) not inspired by Clinton? If not, could another leader, like David Petraeus, be "the man on horseback" Eliot Cohen warns against but at least some Americans are yearning for? Would a serious third-party candidate fracture the anti-Clinton vote, handing her a bigger victory in November? Or could a three-way election in which no candidate receives 270 electoral votes throw the task of choosing the president to the House of Representatives?

6 Will Trump do something smart like inviting John Kasich or Kelly Ayotte to be his vice-presidential nominee?

Adding a serious leader from the governing wing of the party could at least tempt some doubters to move to Trump on the grounds that he would have a deputy alongside him in the White House of high calibre and even temperament. But history shows that vice-presidential nominees do not swing elections. And many stars of the Republican Party, like Nikki Haley and Paul Ryan, have already ruled themselves out.

Establishment Republicans may pray that Trump gets blown out of the water in November, discrediting his cause. But the anger and passion motivating his voters, and those of Sanders, will not soon subside.