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The health care plan was signed into law in 2010 and has drawn both praise and criticism since then.
Gallup reports that the shift was most noticeable among independents, with their approval going from 40 percent in November to 57 percent in the latest poll. Approval among both Democrats and Republicans increased 10 percent during the same period.
A Republican bill to repeal and replace the measure last month were thwarted when the legislation was pulled shortly before voting, at the direction of President Trump when it became clear it would not pass. Only 30 percent of Americans want to repeal the law and replace it, according to the poll.
Gallup has been polling Americans on their views of the Affordable Care Act since 2012, and Gallup's latest poll, released Wednesday, showed that 55 percent of respondents approve of the ACA.
That number is up from the 42 percent who approved of the law immediately after the 2016 elections, with 53 percent disapproving.
Approval of the ACA previously peaked at 48 percent twice in Gallup polls: in the first survey on it, in November 2012, and in mid-2015. Wednesday's poll was the first time that a majority of respondents approved.
Even though a majority of respondents now approve of the ACA, that doesn't mean that they think it shouldn't be fixed.
Only 26 percent of Americans want to keep the law without adjustments, and 40 percent want to keep it in place but make significant changes.
Those answers fall largely along party lines. Slightly more Democrats (47 percent) want to keep the ACA but make changes than want to keep it without major changes (44 percent).
Among Republicans, 60 percent want to repeal and replace the law.