Minnesota state representative Ryan Winkler, a white Democrat, did not have the world's most well-thought-out response to the Supreme Court's 5–4 ruling against a key provision of the Voting Rights Act today. As you can see in the tweet above, he referred to Clarence Thomas, who voted with the majority, as "Uncle Thomas." Uncle Tom is a derogatory term used to describe "a black who is overeager to win the approval of whites (as by obsequious behavior or uncritical acceptance of white values and goals)," which is exactly how it's being used here by Winkler.
Winkler deleted the tweet and apologized, but also claims, dubiously, he didn't realize "Uncle Tom" had "racist" connotations.
To be fair, there are situations when it is not racist for a white guy to call a black guy "Uncle Tom," such as when referring to an actual uncle of his named Tom, which is not what happened here, so never mind.