By Jason LangeWASHINGTON, Aug 24 (Reuters) - U.S. public approval of the war with Iran fell to its lowest level since the conflict's early days, helping to hold President ‌Donald Trump's popularity at a record low, according to a Reuters/Ipsos poll that closed on ‌Monday.The four-day poll showed just 31% of Americans support U.S. military action against Iran, down from 37% in a March Reuters/Ipsos ​poll and 34% earlier this month. The decline was driven by fewer self-identified Republicans backing the conflict. Some 69% of Republican respondents support the war, compared with 77% in March.The war has been dragging on Trump's public standing this year, and for the second survey in a row just 33% of respondents ‌said they approved of the Republican's ⁠performance in the White House, the lowest level in Reuters/Ipsos polls in Trump's first or second term.Trump has told Americans that the war - which started with ⁠U.S.-Israeli attacks on Iran on February 28 - is necessary to keep Tehran from developing a nuclear weapon. The conflict has cooled in recent months but Iran has kept in place a de facto blockade of oil exports ​from ​the region, which has pushed U.S. gasoline prices near ​record highs.Trump has vowed to ramp up ‌economic pressure on Iran and Treasury Secretary Scott Bessent on Monday announced a possible expansion of sanctions on countries doing business with the country, but stopped short of actually imposing penalties, a sign of how thorny the conflict has become for the administration.FEARS OF A LONG WARSome 83% of the country thinks the war will go on "for an extended period of time," the latest Reuters/Ipsos poll found, ‌up from 80% earlier this month.Trump campaigned ahead of the ​2024 presidential election on promises to control inflation and avoid ​lengthy military conflicts. Six months into the ​war with Iran, U.S. gasoline prices are more than a dollar higher per ‌gallon than they were before the war. ​That is weighing on Trump's ​Republican allies who will be defending narrow congressional majorities in the November 3 midterm elections.Independent voters in the poll favored Democrats over Republicans - 33% to 19% - when asked who they ​would vote for in congressional elections ‌if the election were held today.The poll surveyed 1,215 U.S. adults nationwide and had ​a margin of error of 3 percentage points in either direction.(Reporting by Jason Lange in ​Washington; editing by Scott Malone and Deepa Babington)