House and Senate Republicans seeking to reconcile the differences between their tax bills this week will face the harsh reality that not all of the tax breaks in the two bills can be signed into law, and that the GOP will have to choose which ones live and which ones die.

Congressional Republicans and President Trump have momentum on taxes, following the Senate’s passage of the Tax Cuts and Jobs Act in the early hours of Saturday morning. But they will quickly run into two constraints as they move to conference on the two bills.

The first is that they are limited to only a $1.5 trillion net tax cut over 10 years, thanks to instructions written into the budget process underlying the tax legislation. Because not all the tax cuts in both bills overlap with each other, not all of them will fit into that $1.5 trillion maximum.

The second is the Senate rule requiring that the bill not add to federal deficits beyond the 10th year.

With those parameters, Republicans face several key decisions:

1. The corporate tax rate

The Senate-passed bill contains the 20 percent corporate tax rate also sought by the House, but it doesn’t kick in until 2019.

That delay means the Senate’s rate cut loses $282 billion less in revenue over 10 years compared to the House bill, according to Congress' Joint Committee on Taxation. But lawmakers will have to decide when the rate starts.

2. The corporate alternative minimum tax

In a last-minute change, Senate Republicans canceled their plans for eliminating the corporate minimum tax and kept it in their bill. That provided them with about $40 billion to apply toward other tax breaks prioritized by senators.

But business groups are not pleased. PwC tax analysts called the move a “notable departure from the tax reform goals” established by Republicans.

It’s likely that Republicans will have to find that money somewhere else in conference.

3. Permanent or expiring tax relief for individuals

To keep the net tax cut below $1.5 trillion, and to ensure that the bill did not add to long-term deficits, Senate Republicans wrote a bill in which all the individual tax breaks phase out. The lower tax rates, the doubled standard deduction, the child tax credit, and others all disappear after 2025.

So too would the new tax breaks for businesses that file through the individual side of the code, so-called "pass-throughs" like partnerships and S corporations.

The House bill, too, has provisions that expire for budgetary purposes, most notably a $300 credit for parents. Overall, though, the individual tax cuts are about $78 billion smaller over the full 10 years in the Senate bill, thanks to the phase-out. As part of the negotiations, House lawmakers may have to get used to the idea of the entire individual side of the reform going away.

“Permanence, particularly of individual tax relief, is obviously a priority of the House and an area where the House-passed bill differs from the Senate-passed bill,” said Marc Gerson, chairman of Miller & Chevalier and a former Ways and Means tax counsel. “But the reality of the budget reconciliation rules prohibiting such permanence will need to be faced if the goal is to enact a tax reform bill in short order.”

“I won’t say it’s make or break but it’s something we’re going to fight for is the permanency of the tax cuts,” said North Carolina representative Mark Walker, chairman of the Republican Study Committee, a group of conservative lawmakers.

4. The death tax

The House Republicans’ bill first doubles the size of bequests that are exempted from the estate tax, then repeals the tax altogether after 2024.

In total, the House bill reduces $151 billion in federal revenues through the lowering and eliminating estate taxes.

The Senate bill would double the exemption, but not eliminate estate taxes. That doubled exemption would phase out in 2025. Those steps cut $70 billion in federal revenue, less than half of the cost of the House bill.

Republicans have long promised small business groups and ranchers that they would repeal what they call the death tax. But a few GOP senators are opposed to dedicating major revenue losses to repeal, setting up a conflict.

5. Individual mandate repeal

Repeal of Obamacare’s individual mandate might be politically difficult, but it makes the math of tax reform easier.

The Senate bill reduces the mandate penalty to zero, a move that actually raises money on paper because the government would spend less on health insurance subsidies as people stopped buying plans through the exchanges or taking up Medicaid.

That frees up $318 billion over 10 years, according to government scorekeepers, which the Senate bill applied to tax cuts.

Some Republicans might blanch at the prospect of a major change to Obamacare, one they failed to reach agreement on earlier in the year. But it’s a top priority for conservatives, Walker said.