The key is how much did Obama’s policies— Obamacare, Obama TAx Cuts, Defense Spending, the Economic rescue plan, etc.— add to the deficit. The CBO does a good job at that. A lot of the debt and the deficits are a result of entitlements like Medicare, Social Security, and Medicaid which is already determined so no President can control it. Obama can own Obamacare— if the deficit grows under Trump, then the CBO will not count that against Trump. If GOP tax cuts increase the deficit (or decrease it) then Trump gets credit for that.