WASHINGTON – U.S. consumer confidence tumbled this month as Americans began to worry that economic growth will moderate next year. But consumer spirits are still high by historic standards. The Conference Board, a business research group, says its consumer confidence index fell to 128.1 in December, down from 136.4 in November and lowest since July. The index measures consumers' assessment of current economic conditions and their outlook for the next six months. Consumers' expectations for the future dropped to the lowest level in more than two years.