Vice President Kamala Harris’ growing lead in the polls may be exaggerated by oversampling Democratic voters and some critics say it’s an intentional bid to bolster her momentum. Democrats are celebrating after Ms. Harris closed the gap in polls with former President Donald Trump and even surpassed him nationally and in some battleground states. Just a few weeks ago, Mr. Trump held a consistent lead in nearly every critical poll in a matchup with President Biden. But Ms. Harris has more...