Washington (CNN) Senate Republicans are working to shore up enough votes within their caucus to pass tax reform next week, even in the unlikely but possible scenario that two Republican senators are absent for the big vote.

Sens. John McCain of Arizona and Thad Cochran of Mississippi have missed votes this week due to health issues . Both their offices say they expect to be back on Capitol Hill for the vote, which will likely happen on Tuesday or Wednesday.

But just in case, Republicans still believe the bill could pass even if they're absent.

Let's do some math

The GOP has a 52-48 vote advantage in the Senate, meaning if all senators are present, they could afford two Republican defections to vote no with Democrats, making it 50-50, and still pass the bill. That's because Vice President Mike Pence can cast a tie-breaking vote, making it 51-50.

Voila. Tax reform is passed.

Any more than two GOP defections, however, and the bill is defeated. Pence is of no use in this situation. This looks increasingly unlikely.

But what if McCain and Cochran have to be absent?

The bottom line is Republicans need a majority of however many senators are present. So if that total number of senators reduces to 98 from 100, then Republicans only need to get to the magic number of 49, rather than 51.

So let's assume all remaining Republicans vote yes and all Democrats vote no. That gets you to 50-48. Done. Passed.

But what if a Republican votes no in this scenario?

That's entirely possible given a few senators have yet to definitively say they'll back the bill.

So let's say McCain and Cochran are absent, and another senator votes no -- and all Democrats vote no. That brings the vote to a tie, 49-49. But remember Pence? He can still cast that tie-breaking vote, making it 50-49. Done. Passed.

The next question -- can two Republicans vote no if McCain and Cochran are gone, and still pass the bill?

No. That would bring the vote to 48-50. The bill is killed.

And if only one senator were absent, how many defections could Republicans afford then?

That would bring the magic majority number needed to 50, rather than 51. Republicans could still afford to lose one Republican to side with the Democrats in that case. The vote tally would be 50-49. Done. Passed.

But they couldn't afford to lose two Republican votes, which would put the vote at 49-50. Bill is killed.

To sum up: If no senators are absent, Republicans can afford to lose two Republican votes and still pass the bill (thanks to Pence's tie-breaking vote).

They can also still pass it if two senators are absent and one Republican votes no (thanks again to Pence), or if only one senator is absent and one Republican votes no.