Has the U.S. Congress gone on record before to condemn such acts of hatred against an ethnic group?

Yes, numerous times.

Two recent examples are:

Japan Resolution 121:
On July 30, 2007, the House passed H.R. 121 recognizing crimes committed by Japan:

Whereas the Government of Japan, during its colonial and wartime occupation of Asia and the Pacific Islands from the 1930s through the duration of World War II, officially commissioned the acquisition of young women for the sole purpose of sexual servitude to its Imperial Armed Forces, who became known to the world as ianfu or “comfort women;”

Resolved, That it is the sense of the House of Representatives that the Government of Japan –

(1) should formally acknowledge, apologize, and accept historical responsibility in a clear and unequivocal manner for its Imperial Armed Forces' coercion of young women into sexual slavery, known to the world as “comfort women,” during its colonial and wartime occupation of Asia and the Pacific Islands from the 1930s through the duration of World War II.

Holocaust Resolution 226:
On June 11, 2007, the House of Representatives passed H.R. 226:

Whereas some 6,000,000 Jews were slaughtered pursuant to Adolf Hitler's diabolical plan for the total extermination of the Jews during the Third Reich, and even more would have perished had it not been for the efforts of a number of United States Government officials who spoke out forcefully against American policy and persuaded President Franklin D. Roosevelt of the need for extraordinary measures to save Jewish lives;

Resolved, That the House of Representatives –
(1) honors the efforts and contributions of those who worked for the establishment of the War Refugee Board and for a more active United States policy to rescue Jews and other victims of Nazi repression who were in imminent danger of death and to provide these persecuted minorities with relief and assistance during World War II; and
(2) commends in particular the actions of Secretary of Treasury Henry Morgenthau, Josiah DuBois, Jr., and John Pehle for their dedication and devotion to helping rescue Jews and other persecuted minorities in the Holocaust.