Many top Republicans have distanced themselves from Mr Trump over a video in which he boasts of groping women.
Mr Trump accused them of disloyalty.
He was particularly scathing about House Speaker Paul Ryan whom he described as a "weak and ineffective" leader.
Addressing a campaign rally in Greensboro, North Carolina, on Tuesday evening, Mr Obama referred to Mr Trump`s crude remarks about women, saying: "Now you find a situation in which the guy says stuff that nobody would find tolerable if they were applying for a job at 7-Eleven."
He said: "You don`t have to be a husband or a father to say that`s not right. You just have to be a decent human being."
Mr Obama questioned how senior Republican politicians could still want Mr Trump to be president.
"The fact is that now you`ve got people saying: `We strongly disagree, we really disapprove... but we`re still endorsing him.` They still think he should be president, that doesn`t make sense to me," he told the crowd.
Mr Obama was interrupted several times by anti-Clinton campaigners but seemed unfazed, saying: "This is democracy at work. This is great."
The hecklers were escorted from the venue by security officials.
More about: