BALTIMORE — Dr. Benjamin Carson was a political unknown just weeks ago. Then with a single speech delivered as President Obama looked stonily on, he was lofted into the conservative firmament as its newest star: a renowned neurosurgeon who is black and has the credibility to attack the president on health care. In his speech at the National Prayer Breakfast last month, Dr. Carson criticized the health care overhaul and higher taxes on the rich, while warning that "the P.C. police are out in force at all times."