WASHINGTON — NATO leaders gathering in Washington starting Tuesday plan to shore up transatlantic support for Ukraine in its battle against Russia. But for host President Joe Biden, the summit has become just as much about demonstrating he is capable of meeting the grinding demands of the presidency for four more years. [time-brightcove not-tgx=”true”] Heads of state from Europe and North America are confronting the prospect of the return of NATO skeptic Donald Trump as Biden tries to save his reelection campaign, which has been in a tailspin following a disastrous June 27 debate performance against Trump. The president said his work at the summit, where NATO is celebrating 75 years, would be a good way to judge his continued ability to do the job.