Incumbent: Heading into the election the incumbent was John Sarbanes (D), who was first elected in 2006. He won re-election on November 6, 2012.
On March 30, 2012 the 3rd district was included in a list released by the National Journal of the top ten most contorted congressional districts, as a result of redistricting.  The district wound through parts of Anne Arundel, Montgomery, Howard, and Baltimore counties.
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The redrawn 3rd district included parts of Anne Arundel, Baltimore, Howard, and Montgomery counties, including parts of the city of Baltimore. On March 30, 2012 the 3rd district was included in a list released by the National Journal of the top ten most contorted congressional districts, as a result of redistricting. 
The new district is composed of the following percentages of voters of the old congressional districts.
In 2012, FairVote did a study on partisanship in the congressional districts, giving each a percentage ranking (D/R) based on the new 2012 maps and comparing that to the old 2010 maps. Maryland's 3rd District became more Democratic because of redistricting.
In 2012, Cook Political Report released its updated figures on the Partisan Voter Index, which measures each congressional district's partisanship relative to the rest of the country. Maryland's 3rd congressional district has a PVI of D+7, which is the 125th most Democratic district in the country. In 2008, this district was won by Barack Obama (D), 61-39 percent over John McCain (R). In 2004, John Kerry (D) won the district 56-44 percent over George W. Bush (R).
As of October 24, 2012, District 3 had the following partisan registration breakdown according to the Maryland State Board of Elections:
"Party advantage" is the percentage gap between the two major parties in registered voters. "Change in advantage" is the spread in difference of party advantage between 2010 and 2012 based on the congressional district number only.