The tax plan has rattled stock markets in recent days as investors worry about the ballooning debt.

Senate Majority Leader John Thune can afford to lose three Republican senators and still pass the bill, and there are more than that, right now, who have problems with it.

The biggest obstacle may be the threatened opposition from Senate conservatives who say the bill doesn’t do nearly enough to cut future deficits, which are projected to exceed $2 trillion annually fo...