Joe Biden was warned on Tuesday by Sir Tony Blair that American intervention in Northern Ireland should aim to “influence not pressurise” or it risks backfiring.
The US president was heading to the province for the 25th anniversary of the Good Friday Agreement and talks with political parties over restoring power sharing at Stormont.
However, the ex-Prime Minister who played a key role in negotiating the peace deal, stressed the need for Mr Biden, who is proud of his Irish heritage, to tread carefully.
He told BBC Radio 4’s Today programme: “You have got to use the American influence on the process with care and with sensitivity.
“I obviously had a very close relationship with President Clinton outside of the peace process but I found him immensely helpful.
“He would immediately understand strategically what was important and what wasn’t.”
He stressed: “The Americans can play an important part on this but you have just got to insert them at the right moment and in the right place.
“There is a difference between influencing and pressurising, and the one tends to be positive, the other can be negative.”
The Democratic Unionist Party is refusing to return to power sharing in Northern Ireland over a Brexit trade row.
Sir Tony explained further: “One thing I learned about the Unionists is if you try and pressurise them to do something that they are fundamentally in disgreement with, it’s usually futile pressure, even if it comes from the US, so you have got to use that influence carefully.”
However, he also stressed: “The American president is still the American president.
“So, deployed in the right way, it’s still helpful.
“Remember, there is a lot of American investment that wants to come to Northern Ireland. That’s also important from the economic perspective.
“If we do it in the right way, the involvement of the American president is positive.”
Before, flying across the Atlantic, Mr Biden stressed his nation’s “commitment to preserving peace and encouraging prosperity” in Northern Ireland.
The US president tweeted: “25 years ago, Northern Ireland’s leaders chose peace.
“The Belfast/Good Friday Agreement ended decades of violence and brought stability.
“I look forward to marking the anniversary in Belfast, underscoring the US commitment to preserving peace and encouraging prosperity.”
Prime Minister Rishi Sunak will meet Mr Biden when Air Force One touches down in Northern Ireland on Tuesday.
The two leaders will hold a bilateral meeting on Wednesday before Mr Biden gives a speech at Ulster University’s new £350 million Belfast campus.
Before his university event, Mr Biden was set to meet Northern Ireland’s main political parties
His visit coincides with the anniversary of the Good Friday Agreement, which largely brought an end to the Troubles, in Northern Ireland in 1998.
He will later travel to the Republic of Ireland, where he will carry out a number of engagements during his four-day stay, including visiting Co Louth and Co Mayo, from where his ancestors hail.
A major security operation will be in place for Mr Biden’s visit, with more than 300 officers from the rest of the UK being drafted into Northern Ireland.
The Police Service of Northern Ireland has also warned of significant traffic disruption in Belfast during the presidential visit, with a number of roads in the city centre already closed.
After he leaves the city on Wednesday, Mr Biden will cross the border to attend engagements in Co Louth.
The president has traced his ancestral roots to the area and he will tour Carlingford Castle in the county before spending the night in Dublin.
He is then expected to visit Irish President Michael D Higgins on Thursday.
It has been announced that Dublin’s Phoenix Park will be closed for 24 hours from 5pm on Wednesday to facilitate the visit.
Mr Higgins’ official residence is within the park’s grounds.
The White House said Mr Biden will take part in a tree-planting ceremony and ringing of the Peace Bell at the president’s official residence, Aras an Uachtarain.
Following that ceremony, he will meet again with Taoiseach Leo Varadkar, whom Mr Biden recently hosted for St Patrick’s Day.
Mr Biden will address the Irish parliament and attend a banquet dinner at Dublin Castle on Thursday evening.
The president’s trip will conclude with a visit to Co Mayo, where he has also connected with distant cousins, on Friday.
He will tour the Sanctuary of Our Lady of Knock and visit the North Mayo Heritage and Genealogical Centre’s family history research unit.
He will then make a public speech at St Muredach’s Cathedral in Ballina.
