Setting up a showdown with the Republican-controlled Congress over the thorny issue of embryonic stem cell research, President Bush vowed to veto a measure, now pending in the House, that would expand federal financing for the studies - an extremely rare personal threat from a president who has never exercised his veto power. The president also expressed concerns about a report that South Korean researchers have developed a method of cloning human embryos to extract stem cells that, theoretically, could be used to develop treatments that are exact genetic matches for patients.