According to conventional wisdom, elections are won by capturing the centre-ground. The successful candidate will attract undecided voters by moderation and painting the other side as extreme and unfit for office.
As has become only too clear over the past few years, this immutable law of politics is now past its sell-by date.
In the run-up to the 2016 US presidential election, followers of conventional wisdom were convinced that not only was Donald Trump heading for a cataclysmic defeat, but he was going to take the Republican party down with him.
It didn't work out that way and, thanks to his dog-whistle approach to electioneering, Mr Trump won, backed by millions of blue-collar workers who deserted the Democrats.
Judging by his twitter barrage aimed at four newly-elected congresswomen, including Somali-born Representative Ilhan Omar of Minnesota, Mr Trump is dusting off the 2016 playbook.
The audience at a North Carolina rally lapped up his attacks on "The Squad" – Ms Omar, Alexandria Ocasio-Cortez, Rashida Tlaib and Ayanna Pressley.
In 2016 the crowd chanted "lock her up" – demanding the imprisonment of Mr Trump's opponent "Crooked" Hillary Clinton.
This time around it is "send her back" – with the Trump faithful demanding that Ms Omar be deported to the country of her birth. Ms Omar is a US citizen.
Mr Trump loved it and one imagines that his campaign team will have been happy too. After all, what they are looking for is an issue to fire up his working-class base and Ms Omar fits the bill nicely.
Over the next few months, it is safe to assume the president will hammer away at the theme in speeches and tweets to his 62 million Twitter followers.
However, there is a danger this could prove to be a spectacular mistake and one which could cost him the White House in 2020.
The biggest threat to Mr Trump is not the evaporation of his vote. There is little doubt his supporters are as energised as they ever were.
But his intemperate racist rants are likely to fire up the other side as well.
My abiding memory of the last election was attending a rally in Portsmouth, New Hampshire, where Bernie Sanders formally endorsed Mrs Clinton as the party's presidential candidate.
It was a carefully choreographed and ultimately futile display of unity. A succession of Sanders supporters told me hell would freeze over before they voted for Mrs Clinton.
According to a survey of 50,000 electors, 12 per cent of people who backed Mr Sanders voted for Mr Trump. A sizeable chunk of others sat on their hands.
It was these voters who cost Mrs Clinton the White House and it is a lesson that should not be lost on Democrat strategists as they consider how to respond to the Trump onslaught on the four congresswomen.
There are already indications that some realise that they are facing a binary choice between disowning the women – who have fallen out with House Speaker Nancy Pelosi – or supporting them.
Politically there is only one option, which is to give them full-throated and unequivocal backing.
To win the White House, the Democrats need to persuade those who sat out the last election to turn out in large numbers, especially in the swing states which by rights they should never have lost in 2016.
Joe Biden, the standard-bearer of the moderate wing of the Democratic party, has got the message.
In his latest fund-raising plea to supporters, his message was unequivocal.
"Four days ago, the president of the United States suggested that four elected members of Congress, all women of colour, ought to ‘go back' to the countries ‘from which they came'.
"And every day since he has repeated this ugly, racist refrain. We've heard it before throughout our history, but it has no place in America in 2019."
Democrats in the House of Representatives have passed a motion condemning Mr Trump's remarks. It may be symbolic, but it will make sure the president's xenophobia takes centre-stage over the next few months.
This should not only arouse those black voters who failed to turn out for Mrs Clinton, but also could increase Democratic support among Asian electors whose anxiety will have been heightened by Mr Trump's rhetoric.
They will fear a subtext – that they are not welcome in Mr Trump's America – and could vote accordingly.
The events of the past few days have done much to change the dynamic within the Democratic party as the race for the nomination intensifies.
Mr Trump has made the party's policy differences shrink into insignificance as activists set their sights on the common enemy, the current incumbent of the Oval Office.
The raucous chanting of the crowd at the North Carolina will no doubt hearten Mr Trump and his army of loyal supporters.
But he has also united the fractious Democratic party against a common enemy and given his opponents the adrenalin shot they desperately needed.
