WASHINGTON (MarketWatch) -- The Congressional Budget Office on Tuesday slashed its estimate of the current year deficit by $200 billion to $642 billion, which would represent the lowest deficit since 2008 and a deficit-to-GDP ratio of 4%. The reduced deficit comes mostly on higher-than-expected revenues and an increase in payments to the Treasury by Fannie Mae and Freddie Mac. The CBO said the debt-to-GDP ratio will fall from around 76% of GDP in 2014 to slightly below 71% in 2018, but then start rising again. By 2023, the CBO says debt will equal 74% of GDP and continue to be on an upward path.