An alliance of nearly a hundred of the nation’s wealthiest donors is roiling Democratic political circles, directing more than $50 million in the past nine months to liberal think tanks and advocacy groups in what organizers say is the first installment of a long-term campaign to compete more aggressively against conservatives. Bernard L. Schwartz, retired chief executive of Loral Space & Communications Inc. and an alliance donor, said the group offers partners “an array of opportunities that have passed their smell test.” This is most helpful, he said, for big donors who lack the time to closely examine their political investment options.