Together with Tony Blair, the President appealed to voters to make a new start, saying they had nothing to lose by giving the Good Friday agreement a try.
In Birmingham for the G8 summit, he declared: "The people are going to have a very interesting, very rich, very good life if they vote to live together. If they vote to stay apart, they are still going to be frustrated, distrustful, angry and a little bit left out."
His comments came against a background of increasing Government anxiety about the Unionist vote in the Northern Ireland referendum, following opinion surveys showing that a majority of Protestants intend to vote No on Friday.
The latest Unionist voice recommending a No vote was that of Lord Molyneaux, David Trimble's predecessor as leader of the Ulster Unionist party, who has joined six of the party's 10 MPs in opposing the agreement.
In his intervention, President Clinton stressed: "If this agreement is embraced, anybody who returns to violence is never going to be a friend of the United States. We won't tolerate it, we won't support it, we will do everything we can to affirmatively oppose it.
He recalled the late Israeli prime minister Yitzhak Rabin telling him in 1993, before signing an agreement with the PLO: "You don't make peace with your friends, you make peace and then you make friends." He said the people of Northern Ireland "can get over this. It's a little bit of a leap of faith but the risks of doing it are so much less than the risks of walking away. Why take the risk that this moment won't present itself again for another generation?"
The Government will be hoping that his words, together with a third visit which Mr Blair is due to make to Belfast this week, will change the pattern of recent weeks, which has seen a steady growth in the number of Unionists intending to vote No.
Mr Blair himself sought to soothe Unionist opinion with assurances on the future of the RUC, the position on accelerated prisoner releases and the arms decommissioning issue.
He said of the possibility of an adverse referendum result: "We are here and we will try and pick up the pieces as best we can. We pick up the pieces when everything goes wrong, but we would be in a situation where it wouldn't be the status quo."
Meanwhile, a large car bomb left outside an RUC station in the town of Armagh was made safe yesterday by army experts. The device, which contained 760lb of explosives, is assumed to be the handiwork of a dissident republican faction.
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