By David Morgan (Reuters) -Lael Brainard, one of President Joe Biden’s nominees to the Federal Reserve, was poised to become the U.S. central bank’s next vice chair on Tuesday, but COVID-19 cases among Democrats in Congress muddled the prospects of a second Fed nominee without similar bipartisan support. The Senate was scheduled to hold a final confirmation vote on Tuesday at 2:15 p.m. ET (1815 GMT) on Brainard, a current Fed governor. Eight Republicans joined Democrats in voting 54-40 on Monday to end debate on Brainard’s nomination. The Senate was also poised to take first steps toward a con…