He was repeatedly reminded. And yes, he knows who Spencer is:
Born in Boston in 1978 to an ophthalmologist Rand Spencer and his wife Sherry, Spencer popularized the term “alt right” to define politics that combined racist White nationalism with isolationism and criticism of mainstream conservatism. Spencer grew up in Texas, Virginia and North Carolina and met President Trump’s senior adviser Stephen Miller at the Duke Conservative Union.
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Spencer supported Trump before the 2016 presidential election and hailed the latter’s decision to appoint Steve Bannon as the chief White House strategist, saying Bannon would be the “best possible position” to influence policy. Bannon did not last in his post even seven months. Praising Trump, Spencer also said then: “No matter what happens, I will be profoundly grateful to Donald Trump for the rest of my life.” he shot to national fame in 2016 after shouting “Hail Trump!” and being greeted with typical Nazi salutes at an event in Washington shortly after Trump’s re-election.
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Avoiding crude slurs, something that Trump hardly cares about, was another reason why Spencer attracted national attention at the time. Last November, however, Spencer was apparently caught using racist slurs on tape against African-American and Jewish people. The audio was released by former Breitbart writer Milo Yiannopoulos on YouTube.
For many, Spencer’s endorsement of Biden was comparable with that of former Grand Wizard of the Knights of the Ku Klux Klan, David Duke, for Trump. On the campaign trail in 2016, Trump was asked time and again about Duke’s support but he kept on insisting that he had no idea about Duke. He even said on one occasion that he first needed to do research on such groups and would disavow if he found there was something wrong. He eventually disavowed Duke’s support for him but the latter remained loyal and even wanted Fox News anchor Tucker Carlson as Trump’s deputy instead of Mike Pence.
I think most Americans are disgusted with Spencer and his racists outbursts against blacks and Jews. But he has a following and Trump thinks some of them are "very good people."
I'm guessing that Spencer has figured the best thing he can do for Trump is endorse Biden. But I kinda doubt if any black or Jewish voters are going to be fooled.