Robert F. Kennedy Jr. is polling stronger than any third-party candidate has in decades, pulling in roughly 10 percent of registered voters across the battleground states as he saps support from both President Biden and former President Donald J. Trump, a new series of polls has found. The overall results in the Biden versus Trump contest were virtually unchanged when Mr. Kennedy was included in the polls conducted by The New York Times, Siena College and The Philadelphia Inquirer. But...