The party that voted to pass a constitutional amendment to enshrine slavery permanently into the US Constitution was the "Republicans."
Look up the Corwin Amendment, which passed the Republican controlled House and Senate by a 2/3rds margin.
So they are lying to you. They aren't telling you the truth.
The Corwin Amendment, also known as House Joint Resolution 80, was a proposed amendment to the United States Constitution that emerged in 1861, right on the cusp of the American Civil War.
It was passed not because the Republicans WANT slavery to continue, but to appease Southern states and avert the war that would have killed and maimed millions of Americans by guaranteeing the federal government wouldn’t interfere with existing state laws, including those concerning slavery.
If ratified, the amendment would have prohibited Congress from abolishing or interfering with any state’s “domestic institutions,” including slavery, through future amendments.
However, the outbreak of the Civil War and secession of Southern states prevented the necessary ratification process from taking place. The amendment never officially became part of the Constitution.
Ther point was it was a desperate attempt to preserve the Union and avert the war, even through compromise on the sensitive issue of slavery.