Isn't Merrick Garland too conflicted to be Attorney General because he is a former Chief Judge on the US Court of Appeals for the D.C. Circuit?
After his Chief Judgeship, Garland was nominated by and is serving under a Democratic President, Garland as Attorney General manifests that he's most probably Democratic. Then some people can regard his previous judgements with a Democratic slant. Or worse, they can tie his previous judgments with Democratic Party, and/or regard his previous judgments as judicial activism for the Democrats.
If Garland hadn't worked for any executive branch after his judgeship, then we might have never known his political affiliation. The party that nominated a Judge of the US Court of Appeals can differ from the judge's preferred political party. I know judges to US Court of Appeals must be nominated by U.S. President. But just because Bill Clinton appointed Garland, doesn't prove Garland leans Democratic.
Here are two examples. George H.W. Bush appointed David Souter to the U.S. Court of Appeals for First Circuit, then to SCOTUS. But Souter's politically progressive judgments make him more Democratic than Republican. Richard Nixon appointed John Paul Stevens to the US Court of Appeals for the Seventh Circuit, then Gerald Ford appointed Stevens to SCOTUS. Again, Stevens's political progressive judgments entitle you to argue that Stevens is Democratic.
I see no modern equivalent to Garland on the list of U.S. Supreme Court justices who also served in the U.S. Congress or on the list of people who have served in all three branches of the United States federal government.