The most recent Gallup poll shows that Americans favor repeal by a margin of 6 percentage points (46 to 40 percent) – and that's even without screening for likely, or even registered, voters.
To put the importance of not screening for likely voters into perspective, in its final generic congressional ballot poll of registered voters before last November's election, Gallup showed Republicans and Democrats to be in a dead-heat: 46 to 46 percent. When Gallup switched to polling likely voters just one week later, it showed Republicans leading by a margin of 13 points (53 to 40 percent) in a high-turnout election (and by even more than that in a low-turnout election). So screening for likely voters increased the GOP's margin by 13 points.
Given that Gallup isn't screening for registered voters this time around, let alone likely ones, even a conservative estimate would suggest that Gallup would find the margin of support for repeal among likely voters to be at least 19 points (6 points among all adults + something for screening for registered voters + 13 points for screening for likely voters). That would put Gallup's numbers closer to those of Rasmussen, which show that likely voters support repeal by a margin of 22 points (60 to 38 percent) and that likely voters who are independents support repeal by a margin of 36 points (66 to 30 percent).