The math behind repealing and replacing the Affordable Care Act

— -- Now that Republicans have rolled out their legislative plan to repeal and replace the Affordable Care Act, House Speaker Paul Ryan and other Republican congressional leaders have begun to sell the bill to build the support required for its passage.

House of Representatives: 216

In the House of Representatives, Republicans must amass a simple majority to pass the legislation. While that threshold is typically 218 ayes when all House seats are filled, that number has decreased to 216 because of five vacancies. It could change if there are additional resignations or if any of the vacancies are filled through special elections before a vote.

The magic number: 21

Since there are currently 237 Republicans in the House and none of the 193 Democrats are expected to vote in support of the GOP’s plan, Ryan can afford to lose up to 21 of his Republican colleagues before granting concessions to any conservatives whose concerns have inspired them to withhold their support. A 238th Republican could be added if new CIA Director Mike Pompeo's former Kansas district remains in GOP control after its April 11 special election, in which case the magic number would become 22.

While it currently would take only 21 House Republicans to kill the bill or force leadership into changes and several are already pressing leadership to make additional changes, Ryan predicted the bill will ultimately pass.

“I have no doubt we’ll pass this, because we're going to keep our promises,” Ryan told reporters on Tuesday, adding that every congressional Republican as well as President Trump “made a promise to the American people and the promise we made to the American people is we're going to repeal and replace Obamacare.”

Senate: 50

In the Senate, no Democrats or independents are expected to vote in favor of the GOP’s plan. But Republicans are expected to vote on the plan under a procedural mechanism known as budget reconciliation, which also requires only a simple majority for passage and bars filibustering. Senate Majority Leader Mitch McConnell has the luxury of a Republican White House, where Vice President Mike Pence’s role as president of the Senate affords him the exclusive right to break tie votes in the upper chamber.

The magic number: 2

With 52 Senate Republicans, if three or more conservatives vote against the bill, Pence won’t have a chance to cast a tie-breaking vote.

Already, Sens. Rand Paul and Mike Lee have expressed their strong opposition to the current state of the bill, placing its success in jeopardy. Sens. Susan Collins, R-Maine, and Ted Cruz, R-Texas, also have deep concerns about the legislation.