Senator Obama has now taken an insurmountable lead in pledged delegates.  The only way for Senator Clinton to win the democratic nomination is for super-delegates to overturn the will of the voters -- which would be a travesty of insider politics over the wishes of the people. 

Senator Clinton's arguments about "electability" (Obama generally performs better in polls against McCain) and "winning the popular vote" (maybe, if you ignore all the caucus states) simply do not hold water.