It depends on how you define "war".
Formal Declarations of War
This is easy enough to verify by looking at the votes for formal declaration of war by the US Congress.
- War of 1812: Senate 19-13, House 79-49. 38.75% opposed
- Spanish-American War: Senate 42-35, House 310-6. 45.5% of the Senate opposed, worth nothingnoting.
Post-WWI you get near-unanimous votes for war.
Undeclared Wars Which Required a Congressional Vote
The US has lots of these and it's unfair in the modern era to exclude them. Formal declarations of war by the US ended after WWII. Instead of a declaration of war they require an Authorization for Use of Military Force (AUMF) from Congress to fund and prolong the president's initial military decision. It's a war. We can look at those votes.
- MNF Lebanon: Senate 54-46, House 253-156. 39.7% opposed
- Gulf War, 1991: Senate 52-47, House 250-183. 43.2% opposed
- Iraq War, 2003: Senate 77-23, House 296-132. 29.4% opposed.
Congress voted on the Vietnam War with the near-unanimous Gulf of Tonkin Resolution giving the president authority
to take all necessary steps, including the use of armed force, to assist any member or protocol state of the Southeast Asia Collective Defense Treaty requesting assistance in defense of its freedom