It's often difficult to define historical figures by modern political terminology. However, if we define a conservative as "someone who seeks to preserve existing social structures" and a liberal as "someone who seeks to reform existing social structures", then I think we can safely call Ulysses S. Grant a "liberal".

As President, Grant advocated for one of the most ambitious programs of social change in American history - namely Reconstruction. The Reconstruction policies supported and defended by President Grant attempted to ensure newly enshrined civil rights for black Americans - including citizenship and the right to vote. During his two terms in the White House, Grant was ready and willing to use the military to enforce the rights of black Americans, and he aggressively fought back against the racial terror perpetuated against black Americans by groups of white Southerners, including the Ku Klux Klan.

In his biography of Ulysses S. Grant (simply titled Grant), biographer Ron Chernow writes:

Grant deserves an honored place in American history, second only to Lincoln, for what he did for the freed slaves. He got the big issues right during his presidency, even if he bungled many of the small ones. The historian Richard N. Current, who also saw Grant as the most underrated American president, wrote: "By backing Radical Reconstruction as best he could, he made a greater effort to secure the constitutional rights of blacks than did any other President between Lincoln and Lyndon B. Johnson." In the words of Frederick Douglass, "That sturdy old Roman, Benjamin Butler, made the negro a contraband, Abraham Lincoln made him a freeman, and Gen. Ulysses S. Grant made him a citizen."

Much of the progress made on racial justice and black civil rights was rolled back after the end of Grant's presidency. The Compromise of 1877 led to the withdrawal of federal forces from the Southern states, which began to impose new racially discriminatory laws, leading to new rounds of racial terror and an era of renewed oppression for black Americans. Many of the basic civil rights established for black Americans during Reconstruction would not be protected by the federal government again until the successes of the Civil Rights movement of the 1950s and 1960s, nearly 80 years after the end of Grant's presidency.