No, I don't mean the Democratic majority in the Congress.
When I took another look at photos of the meeting of The Club of presidents at the White House yesterday, something occurred to me. After Obama's inauguration, a majority of them will be Democrats. The Club will have five members: Democrats Obama, Carter and Clinton, and the two George Bushes, both, of course, Republican.
That has not been true since Harry Truman's death on December 26, 1972. From March 28, 1969 (Dwight Eisenhower's death) to December 26, 1972, The Club consisted of Democrats Truman and Lyndon Johnson, and one Republican, the then-incumbent President Richard Nixon.
There was also a Democratic majority after Herbert Hoover's death on October 20, 1964, until January 20, 1969, when Nixon was first inaugurated. The Club's members during that period were Democrats Johnson and Truman, and Eisenhower, a Republican.
We have to go back a century to find the last previous such period. Some of the pre-Civil War Democrats died during the 1860s, including Martin Van Buren, James Buchanan and Franklin Pierce. By 1869, The Club was split between Democrat Millard Fillmore and the Republican incumbent Ulysses Grant. (I'm leaving Andrew Johnson out of this analysis, because he's difficult to classify.) Not until 1964 was there another Democratic majority.
As I noted earlier, these data are tracked on Wikipedia.
This is mainly attributable to the fact that, during the period that the two-party system as we currently know it has existed, i.e., since the 1856 election, there have been twice as many Republican presidents as Democrats (again, excluding Andrew Johnson).
Up to and including George W. Bush, there have been 18 Republican presidents during that period, and only 9 Democrats. However, those Democrats have been in office for an average of 6.7 years, while the Republicans averaged only 4.9 years. In terms of years, it's about a 60/40 split between Republican and Democratic presidents, respectively.