Whenever a Republican president can lower the deficit, which hasn't happened in 40 years.
Why must it be a Republican? Are you saying you have more confidence in a Republican to accomplish this goal? That's a change of heart for you, Nyvin.
As for your facts, Republican congresses in the late 1990's and 2000's created multiple budget surpluses, and lowered the deficit. Newt Gingrich was responsible for that, laying the groundwork as early as 1994.
During almost all of Clinton's entire term and part of Bush's, the deficit actually went down. Republicans held both houses of Congress from the 104th Congress through 107th.