In their struggles to save the Obamacare reform bill from crashing and burning like it did anyway on Friday, House Speaker Paul D. Ryan repeatedly says this: It's now or never for Republicans to pass health-care reform legislation.

Health-care experts say he's probably right. This moment in time could be Republicans' last, best chance to remake the health-care system.

But President Trump's decision Friday to pull the bill and indefinitely pause their health-care reform efforts is potentially MUCH more damaging than an unfulfilled campaign promise.

A sunk bill could set off a domino effect for Republicans: A bruising intraparty battle on health care could lead to a bruising intraparty battle on tax reform. The inability to pass both of those big-ticket items would deliver a big blow to Trump. These legislative struggles could even cost some Republicans their seats in the 2018 midterm elections.

It all comes down to a little-known budget tool and a clock running out of time to use it.

To pass the legislation quickly (and without help from Democrats), Republicans need to use a special budget procedure called reconciliation that allows them to avoid a 60-vote filibuster in the Senate.

Reconciliation is pretty much made for situations like these: You control the majority in both chambers and want to get something done quickly without having to lobby the minority party for votes. But there's a limit to reconciliation's usefulness, in that Republicans can wield it only while they're debating the budget. And that time is right now. Republicans plan to use the same tool to debate tax reform this fall, the next time they'll be in budget negotiations.

Will the GOP unite to pass health control legislation? (Peter Stevenson/The Washington Post)

Except, Republicans' extremely precise schedule has hit a potential disaster.  By Friday afternoon, more than 35 GOP House lawmakers have expressed opposition to it. That's more than enough to sink it.Even if it could pass the House, the bill faces an even tougher crowd in the Senate, where the margin for victory is thinner: More than 17 GOP senators have expressed hesitation or opposition to it, and GOP leaders could only afford two lose two votes.

Instead of picking up the pieces to start all over, it's possible that GOP leaders and Trump could shrug, say they tried to change the health-care system, and move on to tax reform. (They have a self-imposed strategic schedule to keep to, after all.)

And that probably would be the end of any serious effort by Republicans to repeal the Affordable Care Act.

The ACHA differs significantly from the Affordable Care Act in how it approaches coverage, tax credits and medicaid. (Daron Taylor/The Washington Post)

Here's where we get into the domino effect of one failed legislative effort taking down another. After successfully knocking down a health-care bill they hate, an energized conservative wing of the Republican Party (plus a united Democratic Party) could dig in its heels to try to get what it wants out of tax reform. And the game would start all over again.

Tax reform, another top GOP priority, is facing the same ticking clock that health-care reform is. Republican leaders have decided to give themselves a few months to focus on health-care reform while debating the fiscal 2017 budget, then a few months to focus on taxes while debating fiscal 2018 — with very little time in between.

Sure, Republicans could try again next year to reform health care. But the longer they wait to get anything done, the more intraparty opposition hardens. (On health care, conservatives think the bill wouldn't go far enough and moderates are worried that it would go too far. The lines have yet to be drawn on tax reform.)

All of this is running up against a yet another hard deadline. Next year is an election year for Congress, and even though Republicans are fairly confident that they'll retain control of the Senate, there are enough vulnerable Republicans in the House to make compromise on anything less likely the closer it is to November 2018.

Bottom line: The next few days are Republicans' first and last chance to make good on their promise to reform the Affordable Care Act. If they fail at that, they can kiss it — and possibly any other major reform they want to do in a hurry — goodbye.