I like this CBO analysis that chronicles the annual debt since our balanced budget in 2000. For Fiscal 2010, contributors to the deficit are:
http://www.cbo.gov/ftpdocs/121xx/doc12187/ChangesBaselineProjections.pdf
Bush Tax Cuts in 2001 and 2003 - $190 Billion deficit in 2010.
Bush - Lost income tax due to great recession - $700 Billion
Bush - Interest on the debt increases - 290 Billion (on the half trillion in debt the repubs ran up in 8 years)
Obama Stimulus reduced taxes - $180 Billion deficit in 2010
Obama Stimulus spending - $115 Billion
Bush and Obama, discretionary - $550 Billion (Wars in Iraq and Afghan., plus stimulus bill, about 1/2 to each) Bush was running 300 billion annually before Obama was elected.
That gives
Bush $1.2 trillion of responsibility for last years deficit, plus half of the discretionary (300 billion more), for a total of
1.5 trillion of deficit responsibility.
That gives
Obama $300 billion of direct responsibility for last years deficit, plus half of the discretionary (300 billion more), for a total of
600 billion of deficit responsibility.
(those are the big numbers, there is some noise numbers in there, but they don't amount to much)
Bush is responsible for 75% of last years deficit, Obama for 25% of last years deficit.