While 18 percent is low, it's not as bad as the record-low 10 percent approval rating recorded in February and August of this year. In fact, December's 18 percent ties November's 18 percent for the second-highest rating of 2012.
Since 1974, Congress' approval rating has averaged 33 percent, according to Gallup's data.
The poll found that Republicans' approval of Congress fell slightly to 14% from 16% in November, while Democrats' approval increased slightly to 21% from 19%.
Gallup says the resulting seven-point partisan gap in approval of Congress is the largest measured since June 2011, with the exception of the 16-point gap prior to the election in October of this year.
According to Gallup, "This slight increase in the partisan gap may reflect that Democrats are more positive about their congressional party leaders' handling of the fiscal cliff negotiations than Republicans are about their party leaders."
Independents' ratings of Congress remain unchanged in December at 19%.
Gallup says Congress' job approval "could fall significantly" if the nation's political leaders can't reach a deal to avoid tax hikes and deep spending cuts that are set to take effect in January.
Results for the Gallup poll are based on telephone interviews conducted Dec. 14-17, 2012, with a random sample of 1,025 adults, aged 18 and older, living in all 50 U.S. states and the District of Columbia.