NEW YORK — Now is the summer (vacation) of the president’s discontent. As Donald Trump prepares to leave Friday for his annual August holiday at his lush New Jersey golf club, he’s confronting a storm of crises, at home and abroad, that could set the course for his upcoming re-election bid. With his poll numbers stalled and his ability to rally the country questioned, he’s being tested by an escalating trade war with China that may slow the economy, rising tensions with both Iran and North Korea and, in the aftermath of the latest mass shootings, pressure to act on guns and face accusations of his own role in fostering an environment of hate. The dark clouds are converging as the president’s bid for a second term takes on new urgency. Trump exudes confidence but as the two dozen Democrats eager to take his job sharpen their attacks, the White House — or, for the next 10 days, the clubhouse in Bedminster, New Jersey — will have to mount a multifront effort rooted in maintaining his base rather than trying to expand it. “There are often presidents facing reelection who face an onslaught,” said Douglas Brinkley, presidential historian at Rice…