Sure they did. They passed a join resolution authorizing hostilities and they continued to fund the war after it started.
There is no constitutional "magic language" that equals a war declaration.
They may not have said," We Declare War," but in effect, that is what they did, when they granted the President the powers after 9/11 to go after Al Qaeda et al. Scalia's dissent: Worst of all is the Court's reliance on the legislative history of the DTA to buttress its implausible reading of §1005(e)(1). We have repeatedly held that such reliance is impermissible where, as here, the statutory language is unambiguous...The question was divisive, and floor statements made on both sides were undoubtedly opportunistic and crafted solely for use in the briefs in this very litigation. See, e.g., 151 Cong. Rec. S14257-S14258 (Dec. 21, 2005) (statement of Sen. Levin) http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=000&invol=05-184