House Republicans have largely staged an ideological debate over the American Health Care Act. If Speaker Paul Ryan and President Donald Trump manage to win that debate (and House passage), the bill will shift swiftly to the Senate, where Majority Leader Mitch McConnell has promised to move directly to floor action with no committee process or bipartisan negotiations.

He will then find himself dealing with a very different kind of debate than the House had — one that’s primarily about the wisdom and morality of depriving millions of needy people of their health insurance coverage.

That’s because even though the GOP holds slim majorities in both chambers, the geographic bases of those majorities are different, and the dynamics of the 2018 midterms exacerbate those differences. They paint Republicans into a corner: Ryan, from his perch in an upscale Wisconsin district and ever mindful of the need to corral potential Freedom Caucus rebels, crafted a bill whose passage would be devastating to states that Republicans represent and hope to represent in the United States Senate.

The House is debating ideology; the Senate will debate coverage

The bulk of stated objections to AHCA in the House come from right-wing members who complain that it doesn’t genuinely repeal the Affordable Care Act root and branch.

And indeed, the law does keep in place the basic structure of Obamacare’s three-legged stool. Individuals who don’t receive insurance through their job or through a government program will still be able to shop for a plan on regulated state exchanges that bar denying coverage to people with preexisting medical conditions. Financial help to afford a plan will still be available in the form of refundable tax credits. And the individual mandate is replaced with a quasi-mandate that financially penalizes people who don’t maintain continuous insurance coverage.

Critics lambaste this as “Obamacare lite,” and in a purely structural sense that’s exactly what it is. Ryan’s camp counters that the provision of tax credits to purchase health insurance is an idea with considerable GOP heritage. (That, of course, is exactly what Obama said when selling the plan in 2009, but Ryan was unimpressed at the time.)

This is an interesting sort of conceptual argument about the conceptually most interesting part of the ACA. But in a practical sense, a huge share of ACA coverage gains ended up coming from the conceptually boring idea of spending a bunch of money on a huge expansion of Medicaid coverage. And in a practical sense, the Medicaid cuts contained in the AHCA are so large that it would actually create more uninsured people than a straight repeal bill. This massive coverage loss has bothered a few House Republicans — most notably Ileana Ros-Lehtinen of Florida — but the vast majority of the objections and arm twisting have focused on the conceptual issue.

The Senate is different. There, a handful of members have made noises about concerns along the lines of the Freedom Caucus, but a much larger bloc is talking about coverage. Four GOP senators — Rob Portman of Ohio, Shelley Moore Capito of West Virginia, Cory Gardner of Colorado, and Lisa Murkowski of Alaska — have written to McConnell to express concern about sharp rollbacks to Medicaid. Bill Cassidy of Louisiana and Susan Collins of Maine, meanwhile, have actually gone and written their own replacement bill, one that would likely lead to much smaller coverage losses.

House and Senate geography look different

If you look at the Center for American Progress’s handy district-by-district breakdown of coverage loss under AHCA, you’ll see that the hardest-hit districts are often majority-minority ones represented by Democrats. Indeed, of the 100 hardest-hit districts, only about a quarter have GOP representation. And if you look up the most vulnerable House Republicans — generally suburban districts whose white population is very well-educated that swung away from the GOP in 2016 — they tend to be spared the worst of the bill’s sting.

Conversely, many House Republicans districts are so safe they wouldn’t lose to a Democrat even if the Trump administration unleashed a nuclear holocaust. To the extent that they worry about anything, they worry about losing their reputation for total ideological purism — and inviting a possible primary challenge from the right.

By contrast, Arkansas, Kentucky, and West Virginia are the three of the top five states that have been helped by the Affordable Care Act. Louisiana expanded Medicaid relatively late in the game, so it hasn’t yet made it onto the list, but it has the right demographic profile to end up near the top of the rankings once expansion is fully rolled out. Maine, too, is joining the Medicaid party late but stands to gain substantially from it.

And the most vulnerable 2018 Senate Republican, Dean Heller of Nevada, represents a lower-income state with a lot to lose if the AHCA passes. That’s why in addition to the six or so Republicans who’ve raised explicit objections to the House bill, you’ve heard all kinds of murmurs of discontent in the Senate — including from die-hard conservatives like Arkansas’s Tom Cotton.

Coverage has a lot of stakeholders

Beyond geography, one big difference here is that there’s no such thing as a truly safe Senate seat. A Republican in Arkansas can certainly sleep more peacefully at night than one in Pennsylvania, but weird things happen in Senate races all the time. Everyone has to worry at least a little.

But legislators of all stripes worry about representing their constituents. And in the case of health care, that means more than just caring about the low-income people who directly receive the Medicaid benefits. Even Republicans who don’t necessarily prioritize input from poor people are interested in the views of their state’s doctors, nurses, hospital administrators, and other health care providers. And everyone from retail store owners to country commissioners has a general interest in the overall quantity of money that Medicaid expansion has pushed into these key states.

To pass in the Senate, a repeal plan is almost certainly going to need to address these basic financial concerns in some kind of plausible way. That’s going to mean making changes that exacerbate the basic concerns of House ideologues by making sure the bill does considerably less to roll back Obama’s coverage expansion.

In other words, the AHCA isn’t just on thin ice in the House and the Senate — it’s beset by critics who want diametrically opposed changes. It would be foolish to assume that means it can’t pass — politics is unpredictable, after all — but Trump is in many ways further from his objective than it seems.