The "Blue wall" is a term used by political pundits to refer to eighteen U.S. states and the District of Columbia that the Democratic Party won in each presidential election from 1992 to 2012. George W. Bush, the only Republican president elected during this time, was able to narrowly win the electoral college in 2000 and 2004 only by winning states outside of the blue wall, enough to defeat his Democratic opponents, Al Gore and John Kerry, respectively.
During the 2016 presidential election, many political pundits speculated that the "blue wall" made Hillary Clinton a heavy favorite to win the electoral college. However, Republican nominee Donald Trump was able to achieve victories in the three blue wall states of Michigan, Pennsylvania and Wisconsin, as well as an electoral college vote from Maine, a fourth blue wall state. He was consequently elected president with 306 electoral college votes (excluding two faithless electors).
In the 2020 United States presidential election, Democratic nominee Joe Biden defeated President Trump by reclaiming Wisconsin, Michigan, and Pennsylvania for his party. Trump's lone breakthrough in the "blue wall" in 2020 was the one electoral vote from Maine, which he again won. Coincidentally, Biden won 306 electors, the same number Trump did in 2016.
The term "red wall", or "red sea", is less commonly used to refer to states that Republicans have consistently won in previous election cycles; however, most of these states represent significantly fewer electoral college votes than the blue wall. These terms refer to the colors that have become associated with the Democratic and Republican parties, respectively, in the 21st century.
Origin
Ronald Brownstein claims to have coined the term "blue wall" in 2009. After the 2012 presidential election, Paul Steinhauser called "blue wall […] the cluster of eastern, Midwest and western states that have traditionally gone Democratic." The earliest description of the forces creating the blue wall comes from a Houston Chronicle blogger, Chris Ladd. A Republican, Ladd wrote in November 2014 that the seemingly impressive Republican win in the 2014 mid-term elections had overshadowed another trend apparent in the results – a demographic and geographic collapse.
The blue wall referred to a perceived Democratic demographic lock on the Electoral College resulting from the Republican Party's narrowing focus on the interests of white, rural, and Southern voters. According to Ladd, the presence of the blue wall means "a minimally effective Democratic candidate" is all but assured of winning 257 electoral votes, just 13 short of the threshold needed to win the Electoral College and the presidency. Ladd's analysis became popular when MSNBC commentator Lawrence O'Donnell featured it on a post-election episode of his show The Last Word with Lawrence O'Donnell.
A similar "red wall/sea", behind which lie states solidly Republican, has also been posited to exist. But, having fewer electoral college votes, it would be theoretically easier for a Democratic presidential candidate to win without breaching it, as had been done in 2012.
States behind the blue wall
Behind this "blue wall" lay states, many carrying a relatively high number of electoral votes, which appeared to be solidly behind the Democratic Party, at least on the national level, and which a Republican presidential candidate appeared likely to have to write off, seeking a total of 270 electoral votes from other regions. States behind this wall lay generally in the Northeastern United States, and the West Coast of the United States, and included some of the Great Lakes states. In each of the six presidential election cycles prior to 2016, the Democratic Party had won 18 of these states (as well as the District of Columbia), totaling 238 of the necessary 270 votes need to win. The "big three" Democratic stronghold states include California, New York, and Illinois.