From an interview with Bruce Oudes in 1989 [emphasis mine]:

"Lou Harris, of course, is a pollster and he was putting out some polls in '70 and '71, I believe it was, that were very -- that Nixon thought were rather unfair, didn't cast Nixon in as fair a light as possible. So what happened was a campaign to put a bit of political arm on Mr. Harris. He was stroked on the one hand. The trip to Moscow, invitation to the White House, that kind of thing. But by the same token, it is very clear in these memos and materials that he was also threatened by the loss of the contracts that he held with some government agencies. And it is certainly for Mr. Harris himself to say how much he was or was not influenced by all this pressure. But you see from the documents themselves the fact that there was considerable pressure. And that he certainly in his responses to the White House seemed to change his attitude quite substantially before the '72 campaign."