That's only partially true. Bill Clinton's poll numbers may have been holding but Clinton had worn out his welcome after eight scandal-filled years. The last-minute pardons hurt him, as did the trashing of the White House. If he was so popular, why did Al Gore avoid using him to bolster his own candidacy? That decision tells me that Clinton was not truly popular by 2000, just tolerated. Nixon had good approval numbers too, right up to the day he resigned.
Meanwhile, George Bush has approval numbers Clinton only dreamed of, and Clinton and his minions are still trying to whitewash Bill and prop him up as a success, when every month's events (the mid-east mess, now) show how poor a job Clinton actually did as President. Media shills aside (they'll be forgotten eventually), history won't be kind to the 'Man from Hope'.
Polls still show Clinton with high approval, so the pardons didn't hurt. And the trashing? Bush said it didn't happen. As for Gore not using him, that is not a reflection of his popularity but rather Gores bad judgement. After all, he did lose the electoral votes. Maybe if he had used Clinton, he would have garnered more of the popular vote in some of the states he did poorly in and won the electoral votes in addition to winning the popular vote.
Do you have a source for Nixon approval ratings?
No, he left with an end-of-term rating exactly the same as Ronald Reagan's, ~68%. And unlike Reagan and Roosevelt, that rating dropped almost immediately once the post-January 20 scandals broke. I'm unaware that his numbers ever recovered.
That linked article also makes clear that Jim's correct about the difference between the public's views on Clinton's job performance (which took place during the 90's economic bubble [which started during Bush 41's term], when we were all still in our mindless post-Cold War, pre-9/11 "things will never be bad again" fog), and their view's on Clinton as a human being (only 34%).