* History weighs on conservative leader
* Early critic of bailout, now strongest defender
* Scion of prestigious Greek family
ATHENS, June 15 (Reuters) - Antonis Samaras was once its fiercest critic, but the Greek conservative leader is now the last man standing between the EU/IMF bailout keeping Greece afloat and the leftist leader who would tear it up.
Family history weighs heavily on the 61-year-old as he battles to keep Greece part of the euro zone project, the pinnacle of the continent's drive to unite after World War Two.
His great-grandmother Penelope Delta, a celebrated author of patriotic children's books, committed suicide in 1941, unable to stand the sight of German tanks rolling down the streets of Athens.
Such heritage has sometimes propelled Samaras towards the most right-wing corner of the conservative New Democracy party he has led since 2009.
It saw him resist the cuts in wages and pensions, the tax rises and economic liberalisation pressed upon Greece as the price of international bailouts to stave off bankruptcy, and which led to relations between Athens and euro zone paymaster Germany hitting new lows.
He voted against the first bailout, and then only reluctantly backed the second, worth 130 billion euros. Now he is the deal's most vigorous defender.
"If we cancel the bailout plan we will turn into the black sheep of Europe," Samaras told a campaign rally on Wednesday, imploring voters not to trust his leftist rival Alexis Tsipras.
Leader of the SYRIZA party, Tsipras has emerged virtually from nowhere to challenge for government on Sunday, vowing to tear up the terms of the bailout and call Europe's bluff. He says the euro zone will not eject Greece for fear of the consequences for other indebted members such as Spain and Italy.
Samaras says Tsipras is gambling with Greece's future, and has framed the election debate as a choice between keeping the euro or returning to the drachma and the economic calamity this will almost certainly herald.
"If they feel like playing poker, they should play at home. You don't gamble with Greece," Samaras told the rally.
The scion of one of Greece's most prestigious families, including politicians, authors and national benefactors, Samaras was once the room mate of former Socialist Prime Minister George Papandreou at Boston's Amherst College.
Critics even within his own party say that although Samaras is outgoing and has good communication skills, he is politically secretive and works with a tiny cabal of trusted aides.
In the campaign for the May 6 election, Samaras limited his appearances to carefully staged speeches or pre-recorded chats with ordinary people, hoping to avoid errors that would cut his lead in the last leg of the race.
New Democracy and its fellow-establishment rival PASOK were roundly punished, and SYRIZA emerged in second place.
But this time around, Samaras has swallowed a fear of hecklers, removed his jacket and mingled with voters.
An American-trained economist, Samaras has held several government portfolios, including foreign affairs, since 1989. He defected from New Democracy leading to the party's fall from power in 1993, to found his own party, Political Spring.
But he returned to the fold in 2004 when Costas Karamanlis won elections for the conservatives. He took over as party leader in late 2009, when New Democracy suffered a crushing defeat at the onset of the debt crisis.
"He is a patriot, has been since his youth. In all things, his main yardstick is what is good for Greece, not for himself or his party," said Dinos Arkoumakis, deputy vice chancellor of City University of London and a long-time Samaras friend.