Are you glum about the nation’s prospects? If so, you’ve got lots of company. According to a recent poll for The Hill, a Washington-based daily, 69 percent of American voters say the United States is declining, and 83 percent of voters describe themselves as worried (49 percent say very worried) about the country’s future. Worldwide, the Pew Research Center finds in a separate poll, “many now see the financially-strapped US as a great power in decline.’’ Among respondents in 18 countries, 47 percent expect China to replace the United States as the world’s leading power. Only 36 percent disagree.