"I'd give it a toss-up," the former president told CBS news.
Brother Jeb is "wrestling with the decision" of running for the Republican nomination for president in 2016, George W Bush said.
"I think it's 50-50," the former President said.
"He and I are very close. On the other hand, he's not here knocking on my door, you know, agonizing about the decision. He knows exactly, you know, the ramifications on family, for example. He's seen his dad and his brother go through the presidency. I'd give it a tossup."
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"I'll do whatever he wants," Bush said. "I will be one of his strongest backers. If he wants me out there publicly, I'll be out there publicly. If he wants me behind the scenes, I'll be behind the scenes.... I'm all in for him. He'd be a great president. And the country could use an optimistic view like his," he told CBS' 'Face the Nation' television programme.
"One of the lessons you learn from George H W Bush is that you can go into politics and still be a good father," George W Bush said when asked if it's worth putting a family through a presidential campaign.
"I know Jeb's priority is his family," Bush said. "I also know it's his country. And his deep faith. And he has seen that you don't have to sell those out in order to be a politician.
George W Bush left the White House in 2009 deeply unpopular, beset by lingering wars and a financial crisis. And though his approval ratings have since rebounded significantly, many have wondered how Jeb Bush would deal with his brother's legacy if he decides to run in 2016.
Bush has written a new book, "41: A Portrait of My Father," reflecting on his father's life and public service.
In his book, Bush also brushed aside the argument that he invaded Iraq in 2003 to finish what his father started in the first Gulf War in 1991, when American forces drove Saddam Hussein's Iraqi army out of Kuwait but did not oust Hussein from power in Baghdad.
"I was just responding to kind of the gossip that tends to work around the political circles that clearly, he had only one thing in mind and that was to finish the job his father didn't. Because my dad decided not to go into Baghdad after routing Saddam Hussein out of Kuwait," Bush explained.