Hours after Sen. Hillary Rodham Clinton (N.Y.) began airing an ad in Pennsylvania that subtly questions Sen. Barack Obama's readiness to handle a pending crisis, the Illinois Senator's campaign unveiled an ad of its own that starkly rebuts that charge.

Clinton's ad declares that the presidency is "the toughest job in the world" and asks "Who do you think has what it takes?" It is similar in message to the "3 a.m. telephone call" ad that Clinton's campaign used to great effect shortly before the Texas and Ohio primaries last month.

Obama, as he has done time and again in this campaign when pressed on whether he is ready to lead, falls back on his judgment to oppose the war in Iraq from the start.

"Who made the right judgment about opposing the war and had the courage and character to speak honestly about it?" the narrator of the Obama ad asks.

Then, in a clear shot at Clinton, the narrator adds: "Who in times of challenge will unite us, not use fear and calculation to divide us?"