Prince William and Kate Middleton will exchange vows Friday in a ceremony expected to be watched by almost a third of the planet. But if the story that day will be of a prince and his bride, another will also be playing out behind the scenes: a tale of two kings.

William’s popularity is helping reinvent the monarchy here, with his marriage to a glamorous bride cementing the easy-mannered 28-year-old’s image as the perfect 21st-century king. Yet even as he becomes the single greatest key to ensuring the future of the House of Windsor, many here say William is in danger of overshadowing his far less popular father, Prince Charles, the next in line to the throne.

At stake, royal watchers say, is the public standing of the British monarchy, which during the reign of Queen Elizabeth II has enjoyed virtually unwavering support. Yet despite a relatively successful campaign to improve Charles’s image, an Ipsos Mori poll last week showed a greater percentage calling for William to leapfrog Charles to the throne than at any point since the 1997 death of Charles’s ex-wife and William’s mother, Diana, Princess of Wales. Forty-six percent of respondents now say Charles should step aside.

Britons are likely to get Charles as king, whether they like it or not. The succession of Charles, who as of last week has spent more time waiting for the crown than any other heir in British history, is enshrined by law and unlikely to be changed. Although Elizabeth may be on Facebook and the Friday wedding may be wired for YouTube, royal experts say the monarchy’s hereditary tradition still stands above the whims of public opinion.

But the popularity gap between father and son is nevertheless hanging over the coming reign of Charles III — a man poised to rank among the most controversial monarchs of recent times.

Charles faces a quandary over whether his beloved should receive the title “queen” — in his case, Camilla, Britain’s most famous home wrecker during Charles’s marriage to Diana and now his second wife. At the same time, William will be taking up a new life with Middleton, an elegant young woman of non-noble blood whose skyrocketing public popularity, royal watchers say, is greatly enhancing William’s starlight.

“In an age of celebrity, you can’t deny that glamour counts, and here we have this youthful, popular man in the wings with his new and attractive wife,” said Roy Greenslade, a British media commentator. “So what you’ve got now is the factor of people saying, ‘Gosh, wouldn’t it be nice to have him instead?