What is the 'blue wall'? Latest polls from key states of Michigan, Pennsylvania, Wisconsin

Amid talk of the "bellwether counties" and "swing states," you may also hear about the "blue wall."

Both Vice President Kamala Harris and former President Donald Trump have spent considerable time and resources in the blue wall states throughout their campaigns. The states were Democratic strongholds for decades until Trump flipped three of them in 2016, but President Joe Biden flipped them blue again in 2020.

The blue wall refers to the collection of states that had reliably voted Democrat in recent presidential elections through 2012. Discussion around the so-called blue wall erupted after the 2016 election, when Trump won three such states: Pennsylvania, Michigan and Wisconsin. Together, the states account for a key 44 electoral votes in the 2024 election.