Crossing what was once seen as a political red line, Democrats voted against the usually bipartisan measure, registering dissent about the war in Iran.

Senator Angus King, the Maine independent aligned with Democrats, has never in more than a decade in Congress opposed the annual Pentagon policy measure, a bill that traditionally draws overwhelming bipartisan backing by lawmakers eager to show support for America’s military.

Yet Mr. King this month crossed what was once considered a political red line, by joining Senate Democrats in unanimously blocking a bill that would lay out more than $1.15 trillion in Pentagon spending.

On Wednesday, 205 House Democrats voted against that chamber’s version of the legislation, a striking break from the past when only the most liberal Democrats dared to oppose the annual defense bill.

The shift underscored the hardening partisan divide over the war with Iran and its elevation as an issue in the coming midterm campaigns. It also highlighted an evolution in the strategy of leading Democrats in Congress, who have become more responsive in recent months to pressure from voters to reject the status quo.

“It is hard to go home and say we need this level of increase when I just learned that, for example, over 4,000 kids are losing SNAP benefits in Maine,” Mr. King said, referring to federal nutrition assistance.

The turnaround reflects what Democrats see as strong voter sentiment against the war. It also points to a realization born of the two recent government shutdown fights Democrats waged with President Trump and congressional Republicans that positions that were once viewed as politically treacherous — such as opposing a defense bill, voting against homeland security spending or blocking government funding overall — can sometimes be political winners instead.