A wave of pessimism engulfed Donald Trump’s New York supporters over the weekend as key members of his statewide organization expressed shock at his attack on Ted Cruz’s wife and fears that he is behind an unsubstantiated claim that Cruz had affairs with five women.

“Everyone has been waiting for months for Donald to become more presidential, and unfortunately, he appears to be going in the opposite direction,’’ one of Trump’s most prominent New York supporters told The Post.

“We understood in the early days when he was trying to differentiate himself from the rest of the pack of candidates that he had to do certain things, but why is he now doubling down on that sort of crap, starting with his talking about the size of his penis and now ending up attacking Cruz’s wife and the stuff about alleged affairs?’’ the Trump supporter continued.

A second Republican official, also citing Trump’s attack on Cruz’s wife, Heidi, said, “I wish Trump would just shape up and start acting more presidential.’’

Both sources said they feared that Trump’s actions, which included a threat to “spill the beans’’ on Heidi, would likely cost him support among Republican voters, especially women, in New York’s April 19 primary.

Polls show female voters are increasingly alienated from Trump.

A female Republican activist who hasn’t taken sides in the presidential contest predicted that Trump would suffer “incredible damage’’ if it’s proven — as Cruz has alleged — that he or Roger Stone, the “dirty trickster’’ and Trump’s political strategist, had a hand in a National Enquirer “report” that claimed that unnamed “political operatives’’ were eyeing reports of Cruz’s “five secret mistresses.’’

“Notwithstanding how resilient Trump has proven himself to be, it’ll be a bridge too far to try and destroy someone’s family and then expect to walk away unscathed,’’ said the activist.

She said that both Trump and Cruz were likely to suffer a backlash from voter disgust.

“As long as it’s hanging out there that the charge could be true, I think it’s a pox on both of their houses and they will both suffer from it,’’ she continued.

Trump’s supporters said that despite their own growing concern about their candidate’s conduct, they remained hopeful he would win the bulk of New York’s primary delegates.

“So far neither Cruz nor [John] Kasich even has a campaign going on in New York, so, even with Donald’s troubles and the growing doubts about him, there’s a good chance he’ll win most if not all the delegates on primary day,’’ said another prominent Trump activist.

Bernie Sanders’ blowout wins over Hillary Clinton on Saturday in the Washington (73 percent), Alaska (81.6 percent) and Hawaii (70.6 percent) Democratic caucuses are prompting Democratic insiders to say that Clinton must win New York’s big April 19 primary with an equally large margin — or risk serious damage to her credibility.

“Hillary’s put so many of her chips in New York — her headquarters, being the senator here — that anything less than a 30-point win would be tough for her, and her people are telling everyone as much,’’ said a prominent Democratic operative involved in campaigning.

Many Democrats warn, however, that Sanders, who has only just begun organizing in New York, has the potential of winning a sizable segment of the vote because of the strong “progressive’’ orientation of many of the state’s Democrats.