There have been, for most of the past six months, two kinds of Americans: one group that was confident that President Trump was going to make their lives better, and one group that never believed that. Very few people started in one group and then shifted to the other.

But a crack has formed among the Trump faithful, which threatens to erode his approval ratings if it widens. It appears the wedge creating it is health care — specifically, a fear among some Trump supporters that the Senate bill to repeal and replace Obamacare will end up hurting them personally.

Support for the health care bill tracks closely with approval for Trump and faith that his policies will improve the US economy, Vox/SurveyMonkey polling shows. It is not a complete overlap, however: About one in seven Trump supporters now fear that the Senate health care bill will make them worse off.

Those supporters are the most vulnerable part of Trump’s coalition. They have lost faith in Trump’s promise that he would replace Obamacare with something “much better,” and they have less faith in the rest of his presidency too. Compared with their fellow Trump backers, they are more economically anxious, less confident in Trump’s economic policies, and more concerned about the Russia scandal and the administration’s possible ethical violations.

The polling numbers do not, on their own, show a causal relationship between Trump’s personal approval and the sinking ratings for the Senate health bill — only 15 percent of this month’s poll respondents said would make them better off, down from 21 percent last month. But the drop was most pronounced among respondents who are on Medicaid or purchase health insurance on the individual market — areas that would be affected by the Senate bill. (Overall, only 16 percent of Americans want to see Medicaid cut, as the bill would; 45 percent want to see it expanded.)

Together, the polling numbers hint at a possible pattern for Trump in the months to come: that supporters could waver on him if they perceive his policies are not delivering in the way he promised.

Supporters continue to believe Trump will make good on his policy promises

Trump has accomplished relatively little, in concrete policy terms, to fulfill the broad promises he made to boost job creation, economic growth, and affordability and access for health care. But he has continued to engender a powerful hopefulness among his supporters that he will eventually deliver in each of those areas.

We see Trump supporters’ continued exuberance in surveys of economic optimism, including the monthly Vox/SurveyMonkey Economic Confidence Index, which clocked in at 55 (out of 100) for July, barely changed from January. And we see that exuberance holding Trump’s approval ratings at a low but steady floor for the past several months, settling at 42 percent in the most recent survey.

Approval of Trump is the No. 1 driver of economic confidence in the Vox/SurveyMonkey polling. It is also tightly correlated with expectations about the health care bill. On average, respondents who say they will be better off under the Senate bill report an economic confidence index score of 81. Those who think the bill won’t affect them much have an average index score of 64. Those who think the bill will hurt them have an average score of 40.

The more hopeful you are about the health care bill, the more likely you are to believe the Trump economy is about to deliver for you and the nation. Seven in 10 of the respondents who expect to benefit from the Senate bill also believe their families will be better off financially in a year; 84 percent believe the country will experience “continuous good times” over the next five years.

That group of health care optimists makes up about one-third of Trump supporters. About half of Trump supporters say the bill won’t change their lives either way. But here’s what’s important: About 14 percent of Trump supporters believe the bill will make them worse off.

The health care worrywarts are the most fragile part of the Trump coalition

It’s important to keep an eye on that 14 percent, because they seem to be crucial swing voters. Only about half of them identify as conservative or right-leaning; 27 percent say they are independents, and 20 percent say they are left-leaning. Of all the different factions of Trump’s base, this group is the least loyal to the Republican Party.

This group is more worried about the economy, and they are skeptical that Trump will help them prosper. On the Vox/SurveyMonkey index, Trump supporters overall rate the economy at 74, but Trump supporters who are skeptical of the Senate health care bill only rate the economy at 59. (People who disapprove of Trump, by comparison, average a 41.)

Overall, 46 percent of Trump supporters believe the president’s economic policies will help people like them “very much” — but among Trump supporters who are skeptical about the Republican health care bill, only 17 percent agree.

These same people are also more alarmed than other Trump supporters about the recent news of the Trump campaign’s possible collusion with Russia during the election: 30 percent of them say this is a “serious issue,” compared with only 13 percent of Trump supporters overall.

A partial explanation for these patterns has to do with Medicaid. The Senate bill would make large cuts to this health care program for low-income Americans, which would hurt many Trump voters. Trump supporters who disapprove of the Senate bill tend to be poorer than other Trump supporters, and they are much more likely to be on Medicaid: About 13 percent say they rely on Medicaid as their main source of health insurance, compared with only 5 percent of Trump supporters overall.

If you ask Trump supporters who disapprove of the Senate bill, 42 percent say they want Medicaid expanded, not cut. That’s an unpopular opinion among most Trump supporters, who want Medicaid to either stay the same (41 percent) or shrink (31 percent).

Concerns over Medicaid cuts, as it happens, have emerged as a key concern among most of the Republican senators who have not yet committed to vote for the bill. If those senators break against the bill, and it dies in the Senate, a large part of Trump’s base will be upset — but it may preserve Trump’s popularity among a crucial group of supporters who are already skeptical of the president.