Truman feels himself beholden to his Southern base, waits until after the 48 election, somehow he loses reelection, Dewey comes in, desegregates the Army.
Challenge done.
As an aside here, desegregating the military and supporting the Dem Convention's civil rights plank probably won Truman the election by shifting enough black votes toward the Democrats to win the crucial 3 states of Ohio, California and Illinois that swung the electoral college to Truman. While I don't doubt that Truman was genuinely disturbed by reports of returning black war veterans being mistreated affecting his views on civil rights, Clark Clifford's 1947 memo on strategy for 1948 covered this in some depth with a focus on the importance of the black vote, meaning that Truman's actions were also about calculating politics:
"(d)
The Negro. Since 1932 when, after intensive work by President Roosevelt, their leaders swung the Pennsylvania Negro bloc into the Democratic column with the classic remark, "Turn your picture of Abraham Lincoln to the all - we have paid that debt", the northern Negro has voted Democratic (with the exception of 1946 in New York).
A theory of many professional politicians is that the northern Negro voter today holds the balance of power it Presidential elections for the simple arithmetical reason that the Negroes not only vote in a bloc but are geographically concentrated in the pivotal, large and closely contested electoral states such as New York, Illinois, Pennsylvania, Ohio and Michigan. This theory may or may not be absolutely true, but it is certainly close enough to the truth to be extremely arguable.
In great measure, this explains the assiduous and continuous cultivation of the New York Negro vote by Governor Dewey and his insistence that his controllable legislature pass a state anti-discrimination act. No less an authority than Ed Flynn has said privately that Dewey will take New York from President Truman in 1948 because he controls the Negro and Italian blocs. This explains the strenuous efforts made by Wilkie in the 1940 campaign to get the Negro vote and it, of course, explains the long, continuing solicitude of the New Deal wing of the Democratic Party toward the Negro.
There are several straws, aside from the loyalty of his leaders to Dewey, that the northern Negro is today ready to swing back to his traditional moorings -- the Republican Party. Under the tutelage of Walter White, of the National Association for the Advancement of the Colored People, and other intelligent, educated and sophisticated leaders, the Negro voter has become a cynical, hardboiled trader. He is just about convinced today that he can better his present economic lot by swinging his vote in a solid bloc to the Republicans. He believes the rising dominance of the Southern conservatives in the Democratic councils of the Congress and of the Party makes it only too clear that he can go no further by supporting the present Administration. Whether his interest lies in a Federal Anti-Poll Tax Statute, in the protection of his civil liberties or in a permanent federal FEPC, he understands clearly that he now has no chance of success with any of these because of the Southern Senators of the Democratic Party.
As well aware of this Democratic chink in the armour as the Negro are the Republican politicians. They make no great secret of their intent try to pass a FEPC Act and anti-poll tax statute in the next Congress. Whether they are successful -- or whether Democratic filibusters will block them -- they can't see how they can lose in such a situation either way. The Negro press, often venal, is already strongly Republican.
To counteract this trend, the Democratic Party can point only to the obvious -- that the really great improvement in the economic lot of the Negro of the North has come in the last sixteen years only because of the sympathy and policies of a Democratic Administration. The trouble is that this has worn a bit thin with the passage of the years.
Unless the Administration makes a determined campaign to help the Negro (and everybody else) on the problems of high prices and housing -- and capitalized politically on its efforts --
the Negro vote is already lost. Unless there are now and real efforts (as distinguished from mere political gestures which are today thoroughly understood and strongly resented by sophisticated Negro leaders),
the Negro bloc, when, certainly in Illinois and probably in New York Ohio, does hold the balance of power, will go Republican." [emphasis added]
In short, an attempt by Truman to cater to the Southern wing of the party was a ticket to defeat.