The word "footprints" does not exist in the text of the short column by Jeffrey Toobin, whose point is only that Justice Thomas hasn't written the majority opinion in significant cases. That doesn't say much of anything about the heft of Justice Thomas's presence. The Chief Justice — or, if the Chief Justice is not in the majority, the senior Justice in the majority — decides who will write the opinion, and the person who assigns the opinion tends to take the most important cases for himself.
Neither Chief Justice William Rehnquist, who presided over Thomas’s first fourteen years on the Court, nor Chief Justice John G. Roberts, Jr., who has run the court for the past eleven, ever assigned Thomas a landmark opinion for the Court... The truth is that Rehnquist and Roberts never trusted Thomas to write an opinion in a big case that could command a majority of even his conservative colleagues.How does Toobin know enough to speak about "trust"? Perhaps he's merely relying on Thomas's predilection for strong originalist principle:
Why was this? It is because Thomas is not a conservative but, rather, a radical... an extreme originalist... guided exclusively by his own understanding of what the words of the Constitution mean... His vision is more reactionary than that of any Justice who has served on the Court since the nineteen-thirties...Originalism and adherence to text is a bad thing in Toobin's view, but to describe it is to destroy the assertion in the headline. These are very distinct footprints.
Thomas was a young man of forty-three when he joined the Court, and he is now sixty-eight. His views, which never really found favor even in the years of conservative ascendancy, appear headed even further from the mainstream....If the marks he's left appear headed somewhere, then metaphorically, they are footprints.