As we all know, the latest CBS news poll shows Bush's latest approval rating at 34%; the Cook Report poll (which has tended to skew a little more pro-Bush than other polls) has him at 40%.
If Bush has indeed fallen below 35%, that puts him in pretty dismal company.
George H. W. Bush occasionally fell into this level during the later stages of '92 campaign (lowest point: 29% in the Gallup Poll in July 31-August 2, 1992).
Reagan's lowest rating was 35% in January 1983.
Beset by inflation and Middle East turmoil, Jimmy Carter fell to 32% in May 1979. He didn't break 35% until the Iran hostage crisis, when he experienced a public opinion rally from November 1979 into the spring of 1980. By June, he was back to 32%; his rating didn't change significantly afterwards.
During his first term, Richard Nixon never fell below 48% (and usually was well above 50%). At the beginning of 1973, the end of American involvement in Vietnam shot his approval rating to 67%. But Watergate sent his popularity plummeting; by August, he was at 31%. He hovered in the thirties for most of the rest of 1973. By December, after the Yom Kippur War (and the ensuing oil embargo) and the Agnew resignation, he had fallen to 29%; he never broke 30 again.
LBJ fell to 35% in August '68, in a poll taken during the Republican convention. During late '67 and in '68, his approval rating mostly hovered in the high 30s and low 40s -- about where Bush is right now.
Truman fell below 35% around the 1946 "Had Enough?" midterms. His ratings during the Korean War were relentlessly awful. He hit 36% in January 1951; he would never again break 35%. In April 1951, after firing MacArthur, Truman fell to 24%.
Bill Clinton, Gerald Ford, John F. Kennedy and Dwight Eisenhower never fell below 35%.
In 2005-06, Bush has had ratings that mostly have resembled those of his father during most of 1992, LBJ in 1967-68, Carter in 1978, or Ford in 1975. If the CBS poll isn't a fluke (and maybe it is -- it was taken during a weekend, and one with a lot of bad news for the president), Bush has fallen into the cellar with the Watergate-era Nixon, the gas-line Carter, and the Korea / McCarthy Truman.
Data from the Roper Center.