Under President Obama, $7.5 trillion has been added to the national debt. The number is being highlighted by the Republican National Committee ahead of President Obama's State of the Union address, which will be delivered tonight from Washington.
As Obama Prepares To Paint A Rosy Picture Of His Presidency, A Look At The Numbers He Won't Mention
$18.1 Trillion:

Total Debt Under Obama As Of January 14, 2014. (U.S. Treasury Department, Accessed 1/14/15)
$10.6 Trillion:
Total Debt When Obama Became President On January 20, 2009. (U.S. Treasury Department, Accessed 1/9/15)
$7.5 Trillion:
Amount Added To The National Debt Despite Obama's 2010 State Of The Union Declaration That He Would Not Leave "A Mountain Of Debt." (U.S. Treasury Department, Accessed 1/14/15; President Barack Obama, Remarks On The State Of The Union, Washington, D.C., 1/27/10)
$4 Trillion:
Amount Of Debt That Obama Once Called "Irresponsible" And "Unpatriotic." (Sen. Barack Obama, Remarks At A Campaign Event, Fargo, ND, 7/3/08)
$1.8 Trillion:
Cost Of ObamaCare's Coverage Provisions Through 2024. (CBO, 4/14/14)
$1.3 Trillion: