As Donald Trump continues to slip in both national and state polls, it appears so does his optimism behind closed doors.

According to a New York Times report, the Republican presidential nominee's mood has become "often sullen and erratic." His associates say he "veers from barking at members of his staff to grumbling about how he was better off following his own instincts during the primaries and suggesting he should not have heeded their calls for change.

Furthermore, Trump has become thoughtful about his relationship with the media — and how it has soured.

He even calls the national chairman of his campaign, Paul Manafort, several times a day "to talk about specific stories."

Trump Saturday reacted strongly via Twitter, criticizing the New York Times for using unnamed sources.

He wrote that he is "enjoying" himself running for president.

It is not just Trump himself that is exhausted and bewildered — his staff is too, according to the piece.

Interviews with more than 20 Republicans that are close to Trump and chose to remain anonymous, they described their nominee as "exhausted, frustrated and still bewildered by fine points of the political process and why his incendiary approach seems to be sputtering."

Jason Miller, a spokesman for Trump, said he is still determined to win the general election and dismissed the notion of any pessimism.

"Behind the scenes we have a very motivated and very focused candidate in Donald Trump, who knows what he needs to do to win this race," Miller said.

Read the full Times report here.