This Thursday (March 23) is a big day for the Trump administration. It’s when the House of Representatives votes on the US president’s first big piece of legislation, the American Health Care Act. This is meant to meet one of his central campaign promises by repealing Obamacare, the landmark health insurance bill the US passed in 201o.

The trouble is that, as one Republican congressman put it, the AHCA may be the most “universally detested” piece of legislation in years. Healthcare experts, doctors, retiree groups, and hospitals have criticized it. It will redistribute wealth from the US’s poorest, oldest, and sickest to the richest, youngest, and healthiest. It’s projected to leave 24 million people without health insurance. Right now, it doesn’t have the votes to pass.

Nonetheless, there is still a chance the AHCA could make it through the House. (The Senate is another matter.) If it does, it will be an interesting case study in how legislation that is not only harmful to many American citizens but also deeply unpopular can be pushed through a polarized Congress by making concessions and issuing threats.

The math

The bill needs 216 votes in the House to advance to the Senate. Democrats, with 193 seats, are staunchly against the AHCA, arguing it would be cheaper and less damaging to fix the flaws in its predecessor, Barack Obama’s Affordable Care Act (ACA).

Republicans control 237 seats. Of those, around 30 are members of the House Freedom Caucus—the party’s hardline wing, many of whom are expected to vote against it. Most of the Republican Study Committee, 172 more traditionally minded lawmakers, are expected to side with it. Another 35 are split.

The Hill yesterday estimated that 28 Republicans were likely or certain to vote no, NBC had that tally at 27, while Huffington Post put it as high as 37. Any tally higher than 21 means defeat for Donald Trump’s bill.

This is Trump’s first big test as salesman-in-chief. How can he and Paul Ryan, the House speaker and the bill’s author, who have been lobbying hard in recent days, get to 216?

The carrots

A “manager’s amendment” introduced on March 20 attempted to placate both the conservative and traditional Republicans.

On the one hand, it gave bigger tax credits to older people on Medicaid, the US’s government-funded health care for the poor. This was an attempt to mollify traditional Republicans who worry AHCA would be bad for their voters.

On the other, it lets states make it harder for people to qualify for Medicaid. It also repeals tax hikes that paid for Obamacare this year instead of next year. All this is to mollify the Freedom Caucus, which wants fewer government benefits and lower taxes.

The White House may be willing to bend a little further, press secretary Sean Spicer indicated on March 21, saying “I don’t want to rule anything out.” The Freedom Caucus is said to want the bill to stop stipulating “essential” services that insurers must offer in their health plans, such as hospital care.

How these changes might affect overall coverage in the US is anyone’s guess. The Congressional Budget Office, which estimated 24 million could lose coverage in the original AHCA, isn’t expected to have time to analyze the amendment before Thursday’s vote.

The sticks

Trump has warned that any Republicans who don’t vote for the bill could lose their seats in the 2018 mid-term elections.”I’m coming after you,” he told Mark Meadows, the head of the House Freedom Caucus. Whether Trump really meant and can carry out that threat is yet to be tested, but in the presidential election he did win all but one county in Meadows’ district in North Carolina. The White House has also pledged to support Republicans who do vote for the bill the bill. “We’re going to make sure that we remember those who stood by us,” Spicer said Tuesday.

Thursday’s vote is deliberately scheduled for exactly seven years after Obama signed the ACA. Since then, repealing it has been a central campaign promise of many Republicans. The bill’s fans see passing a repeal bill, no matter how messy, as politically essential. “Yes, this isn’t a perfect bill, but there is never going to be a perfect bill,” said one Congressional aide to a Republican who supports the AHCA. “This the one shot that we have. It is going to be hard for Republicans to come home and say ‘I had an opportunity to repeal and replace Obamacare and I didn’t vote for it.'”