(Reuters) - Criticism of President Barack Obama's signature healthcare law by a top Senate Democrat this week laid bare post-election tensions that could pose challenges for the party in upcoming fights with Republicans over taxes, energy and immigration. In a high-profile speech on Tuesday dissecting Democrats' losses in this month's midterm elections, Charles Schumer, the No. 3 Senate Democrat, listed "a cascade of issues" botched by the White House, starting with Obama's push for healthcare reforms soon after he took office in 2009. Later on Tuesday, the White House took the unusual step of publicly pledging to veto a deal...