There are plenty of personal reasons why Joe Biden has made this trip to the island of Ireland the longest of his visits as president but it is also just very good politics.
It is hugely important to him personally, no doubt.
The US president talks often and at length about his Irish heritage.
He has said on this visit he is coming home. He feels that strongly. This is his ancestral and spiritual home.
“The best drop of blood in you is Irish,” he says he was told as a young man by his grandfather.
But he also knows the pictures we are seeing from Dublin, and later from County Mayo, play very well back home for a president who has just given his biggest hint yet that he will run for re-election.
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More than one in ten Americans claim Irish roots.
Sometimes, living there, it feels as though it is more than that.
Even Americans with the most tenuous Irish connection seem to make the most of it, especially on St Patrick’s Day.
In a country as young as America, it helps augment people’s sense of history and identity.
The pictures of a president being feted as a prodigal son in Ireland always go down well in America.
A president warmly welcomed on the streets and in the shops of Irish towns.
The images would help any president but Joe Biden especially so.
That is because Biden’s Irishness defines him as both a man and a politician – and has done throughout his half a century in politics.
The common humanity – which he invoked in his speech in Ulster – is an Irish and Catholic value, the president would say.
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At the heart of the president’s brand of politics has always been his decency, his supporters would no doubt argue.
His belief in dignity and duty appeals.
They are all values instilled in him, he says, by his Irish-American family.
And they are values that millions of Americans share and relate to.
Coming here and burnishing his credentials as a genuine son of Ireland strengthens that brand in the minds of voters in the US.
It reminds them why they liked Joe Biden in the first place and why – when it comes to it – they may want to return him to the White House, even in his 80s.
