Semoochie:
Nixon's issues didn't begin with Watergate or even the war. He won in 1968 by a margin of 0.7% of the popular vote, so there was a significant chunk of the population unhappy with the result. In fact, because it was a three-way race with George Wallace getting 9.9 million votes, more people voted for a candidate other than Richard Nixon. He won election with 43.4% of the popular vote.
The Carpenters photo above was taken August 1, 1972, during Nixon's re-election campaign and after the Watergate break-in. By that point, The Washington Post had been reporting on ties between the Watergate break-in and the Committee to Re-Elect the President for six weeks.
On the morning the photo was taken, the Post published a story directly linking money in one of the burglar's accounts to a check from the Nixon campaign.
The Carpenters returned to the White House and performed for Nixon on May 1, 1973---three months after the Senate committee began its investigations and two days after the Post broke the story that three top Nixon aides led the cover-up.
Not saying any of that was necessarily their fault, but despite the landslide election of 1972, a lot of people---especially young people---didn't like Nixon.