WASHINGTON—A substantial increase in personal savings and better than expected tax revenues in many states have weakened the case for President Joe Biden’s sweeping $1.9 trillion economic relief bill, which is now before the Senate. While Biden continues to defend his “American Rescue Plan” to fight the pandemic, a growing number of critics question the size of the proposed spending. During lockdowns last year, many Americans saved more money than usual since most stores and restaurants were closed. And according to a new report by the U.S. Bureau of Economic Analysis, personal income soared by 10 percent in January thanks mainly to government stimulus benefits. The personal saving rate surged to 20.5 percent in January, up from 13.4 percent in December. Americans now have more than $3.9 trillion in savings compared to $1.4 trillion in February last year before the start of the pandemic. “If the next round of $1,400 checks …