Hillary Clinton campaigned Friday in the company of friends and celebrities, first flanked by the billionaire businessman Mark Cuban in Pittsburgh and Detroit, and then at a concert in Cleveland with Jay Z and Beyoncé. High-wattage political leaders fanned out for her around the country: Her husband, Bill, stumped in Colorado, as President Obama rallied voters in North Carolina.

By comparison, Donald J. Trump was a lonely figure.

In the final days of the presidential race, Mr. Trump’s political isolation has made for an unusual spectacle on the campaign trail — and perhaps a limiting factor in his dogged comeback bid.

When it comes to bolstering Mr. Trump, the Republican Party is not sending its best: As party leaders have disavowed him or declined to back his candidacy, Mr. Trump has been left instead with an eclectic group of backup players to aid him in his last dash for votes. Though polls show Mr. Trump drawing closer to Mrs. Clinton, the most prominent Republicans in key swing states still fear that his unpopularity may taint them by association.

Mr. Trump acknowledged the relative bareness of his events at a rally on Friday night: In defiant language, Mr. Trump hailed the size of the crowd packed into an arena in Hershey, Pa.