Basically all the bills to start a war had 90-99% congressional support from what I've seen on Wikipedia. I just wonder if there was any war that 30% or more of Congress opposed starting. I am only asking about the start of the war not its continuation. Wars always have overwhelming support but then wind down. Also exclude cases like Vietnam where there was no congress at all.
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 noting.
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
In addition to the votes to declare war or authorize the use of military force, Congress has voted several times to end a war, by cutting off its funding or mandating the withdrawal of forces.
On December 16, 1969, 73 senators voted for an amendment to a defense-spending bill that cut off any funding for military operations in Laos and Thailand. This bill, with the amendment, was signed by President Nixon. On June 30, 1970, 58 senators passed another amendment extending this to Cambodia. (This was under a different set of filibuster rules than we have today, and contentious legislation could still pass with fewer than 60 votes.) The version that became law was a compromise that prohibited ground troops in Cambodia, but allowed airstrikes.
Several bills to end the war entirely passed the Senate in the early 1970s. In 1971, the Senate passed an amendment demanding withdrawal of US forces, but watered it down to a non-binding, sense-of-the-Senate resolution with the withdrawal date taken out. In 1972, an amendment to a foreign-aid bill passed the Senate that cut off all funding to forces in Southeast Asia except for their safe withdrawal, subject to the return of prisoners of war.
The most famous “amendment to end the war” was the McGovern–Hatfield Amendment (co-sponsored by future Democratic presidential nominee George McGovern and Mark Hatfield, a Republican senator from Oregon). This would have required the war to stop and the orderly withdrawal of US forces by the end of the year, although it would have allowed some troops to remain in the region and made a few exceptions, such as to secure the release of prisoners of war, deal with unanticipated clear and present danger to American troops, and “to provide assistance to the Republic of Vietnam.” On September 1, 1970, the amendment failed in the Senate, 55–39. In January 1971, Gallup found that 73% of the public supported this amendment.
In 1975, Congress refused President Ford’s request for $300M in aid to South Vietnam, just weeks before the fall of Saigon.
The idea of ending a war by de-funding it was revived in 2007, after Democrats opposed to the war in Iraq (but not Afghanistan) took Congress. On March 23, 2007, the House passed a bill requiring withdrawal of all troops from Iraq, 218–212, and the Senate passed it on March 29, 51–47. (As this was a “must-pass” bill funding many military and veterans’ programs, as well as relief for Hurricane Katrina, it was not filibustered.) It was then vetoed by President George W. Bush, and the vote in the House to override the veto received 222 votes, short of the required two-thirds majority.
On November 14, 2007, the House passed the “Orderly and Responsible Iraq Redeployment Appropriations Act” by a vote of 218–203. (Four Republicans voted in favor, and fifteen Democrats against.) This would have required the withdrawal of all troops from Iraq by the end of 2008. The companion bill in the Senate received 53 votes, which under the current rules was not enough to break a filibuster.
A more complete list of major votes on the Iraq war in 2007 was prepared by the Congressional Research Service.