"She understood politics, she followed the game, she was good at it," Daniel said. "She liked living it vicariously through him and she would give him advice, back him up, ask him what was happening. She was by no means running the country from Independence, but she was interested, she was engaged, and he trusted her opinion."
Analyst Roberts said, "The first lady is always something of an enigma. She is such an incredibly influential person, who is not elected and can't be fired. Her greatest influence is her influence on the most powerful man in America. The wife is often the only one that can tell him the truth."
Michelle Obama, who worked as a lawyer, has captured the U.S. public's fascination, with the media covering everything from her fashion sense to her family roots.
Since moving to the White House nine months ago, Michelle has kept a high public profile, hosting cultural events, advocating on behalf of children's health and eating habits, and recently traveling to Copenhagen with the president to support Chicago's bid for the 2016 Olympics.
"As far as the Obamas go," Dallek said, "They seem to be a strong family. It's not only that he has a good marriage, but he's attentive to his children. It resonates with lots of people. He's in a good marriage, he's a family man."
Former first lady Laura Bush also boosted her husband's image, Roberts said.
"One of the reasons people liked George W. Bush was that he was married to Laura Bush. She's very attractive, smart, funny, down-to-earth but very intellectual. The fact that he married someone that substantial made people like him better."
Bush, a former elementary school teacher, advocated on behalf of children's education, human rights and foreign affairs, becoming a vocal critic of Burma's military regime.
And, although then-first lady Hillary Clinton was her husband's "key adviser" and there was "tremendous respect for one another's skills and abilities," Roberts said, the Clintons had a tempestuous private relationship that became public, after the president's affair with a White House intern.
Dallek said, "It was a source of great distress and humiliation for Hillary and landed him in the hot seat by getting him impeached."
"It has an impact on the way people view a president -- if he's known as lacking scruples or is known as being disloyal to his wife. It makes some people quite angry, if a president is known as something other than the straight and narrow. We profess to be a nation with family values," Dallek said.
While the Trumans' relationship was scandal free, Bess Truman preferred to keep her life private.
Grandson Daniel said, "Grandpa was an open book" who saved "every scrap of paper, every bill, every letter."
But his grandmother, he said, was a "very, very private person who thought her business was her own damn business and that was it."
While thousands of Harry Truman's notes and letters are publicly available at the Harry S. Truman Library & Museum in Independence, Mo., Bess burned most of hers in 1955.
"One evening in 1955, around Christmastime, Grandpa came home and found her in front of the fireplace with a roaring fire, throwing in bundles of her letters to him," Daniel said. "And he stopped her and said, 'Bess! Dear God, what are you doing, think of history!' And she said, 'Oh, I have!' and kept throwing letters in the fire. She destroyed almost all of them."