FDR and teamsters?

First of all, thanks. Also, I should let you know that I faked my way through all of high-school American history class. Overall, I understand the role that labor unions played during the depression and WWII. I just don't quite understand why they seemed so important in presidential campaigns. Was it because of financial contributions? Voting power? Strikes? The ideology of the times? What am I missing?

Lastly, although I am particularly looking at the 1944 election, any insight you can provide into the relationship between FDR and the Teamsters would be appreciated.

Thanks again. K-cope.