UNIVERSITY PARK, Texas - Former President George W. Bush says he isn't interested in playing on the national political stage any longer. But for family, he's making an exception.

Asked in an interview with ABC News' Diane Sawyer whether he thinks his brother former Florida Gov. Jeb Bush should run for president in 2016, the ex-president was unequivocal.

"He'd be a marvelous candidate if he chooses to do so. He doesn't need my counsel 'cause he knows what it is, which is 'run,' " the elder Bush brother said about Jeb's possible candidacy, in an interview that first aired Wednesday on "World News with Diane Sawyer." "But whether he does or not, it's a very personal decision."

George W. Bush and his brother Jeb Bush smile while greeting supporters during a campaign rally at Progress Energy Park, Oct. 19, 2004 in St. Petersburg, Fla. (Mark Wilson / Getty Images)

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The former president even allowed himself to picture the potential 2016 matchup: Jeb Bush vs. Hillary Rodham Clinton. It would be a family rematch of the 1992 election, when George H.W. Bush lost to Bill Clinton.

"It'll be a fantastic photo here. It would certainly eclipse the museum and the center," Bush said on the eve of the formal opening of the George W. Bush Presidential Center at Southern Methodist University, just outside Dallas. "I'm interested in politics. I'm, you know, I'm fascinated by all the gossip and stuff that goes on. But the field won't be become clear 'til after the midterms."

Asked for a word of advice to the Republican Party, the former president struck an optimistic note amid rounds of GOP soul-searching: "You will exist in the future," he said with a smile.

On several major issues, though, Bush made clear he's staying away from day-to-day political battles.

With some Republicans calling for immigration reform to be slowed down in the wake of terrorist attack in Boston apparently carried out by two immigrants, the former president brought up his previous support for comprehensive reform but said he wouldn't be commenting on specific legislation.

"I'm a strong advocate in reforming a broken system," Bush said. "It's a difficult issue for members of Congress to deal with. And they're just gonna have to figure out how best to deal with a very complex issue. And I don't know all the particulars of the bill. I do know the system is not working."

Bush also took a pass on the issue of gun control and expanded background checks, which he voiced support for as president.

"There's a lotta issues that people would like to get my opinion on, and I really decided to stay out of the public arena," he said.

He took a similar tack on gay marriage, which Bush opposes - a position that puts him at odds with his former vice president, Dick Cheney; his 2004 campaign manager, Ken Mehlman; and his wife, Laura, and daughter Barbara.

"No, but thank you for trying," Bush said when asked whether he'd like to explain his position in that fast-evolving debate. "I'm not weighing in on issues. … See, you're either in or out."