There really are only two certainties in life. The first is that Republican politicians will pretend their tax cuts will largely pay for themselves, and the second is that Republican economists will largely indulge them in this.

The best recent example of this might be Sen. Susan Collins (R-Maine). On Sunday, she seemed to suggest that, because of all the growth it would supposedly generate, the Senate’s $1.5 trillion tax cut bill wouldn’t actually cost a thing and might even “lower the debt.” Not only that, but Collins said that three top Republican economists — former George W. Bush adviser and current Columbia Business School dean Glenn Hubbard, former Congressional Budget Office director Douglas Holtz-Eakin and former Federal Reserve governor and Bush official Larry Lindsey — had all told her that something close to this was possible.

Now the senator has backtracked a bit to say that she’s not guaranteeing that the cuts would pay for themselves, but she does still think they could. After all, she’s pointed out, the tax bill would just need to make the economy grow an extra 0.4 percentage points a year to cover $1 trillion of its costs.

[After cutting taxes, Trump looking to localities to raise revenue for infrastructure]

There are varying degrees of wishful thinking going on here. First, “no serious economist” thinks that tax cuts fully pay for themselves. Anyone who does is “detached from empirical reality.” That, at least, is what Holtz-Eakin — yes, one of the economists Collins talked to — said in April in a piece analyzing the “rough draft” of the Trump tax plan. I asked both him and Hubbard if they believed otherwise when it came to the Senate bill, and they indicated that they did not. (“They will not pay for themselves,” Holtz-Eakin said, while Hubbard told me that he “had not done a revenue estimate, but the dynamic feedback effect is generally less than 100 percent.”)

Second, most economists don’t think this is anywhere near a close call. “At best, according to the prevailing consensus, the positive feedback effect from tax cuts would recoup in the range of 25 percent to 35 percent of the cost,” wrote, you guessed it, Holtz-Eakin.

Which makes you wonder, then, why he and other Republican economists seem to think that the Senate plan might pay for 40 percent to as much as 67 percent of its costs — perhaps twice as big an effect. I asked them, and their answers weren’t the most rigorous.

[The Finance 202: GOP raids their own corporate rate cut to pay for tax bill fixes]

“The possibility is greater than the typical experience,” Holtz-Eakin told me, “because it has elements that are atypical,” such as the way it would move to a territorial tax system for corporations, allow them to immediately deduct their investments for five years and rebalance the tax treatment of investments funded by debt and equity. Likewise, Hubbard said that while he hadn’t run the numbers, the “feedback effects from corporate tax reform can be larger depending on the type of reform.”

In other words, these are extra special tax cuts. Just take their word for it. You have to because there isn’t really an economic model that tells the same story they do. The only one that comes close is from the right-wing Tax Foundation, but, as I’ve reported, there are serious concerns about its methodology. Mainstream estimates, meanwhile, like the ones from the nonpartisan Joint Committee on Taxation, the nonpartisan Tax Policy Center and the Penn-Wharton Budget Model, show the economy growing just slightly more or very slightly more as a result of these tax cuts — nowhere near the additional 3 percent to 4 percent jump that the Republican economists are predicting.

[Susan Collins is wrong to say that the tax cuts will pay for themselves, despite the economists she cites]

How do they come up with such optimistic numbers? Easy: by making optimistic assumptions. That, as former Obama economic advisers Larry Summers and Jason Furman point out, appears to be what they’re doing. They simply seem to assume the best and then say that if this scenario played out over the next 10 years — which it almost certainly wouldn’t — then these tax cuts really would add 0.3 to 0.4 percentage points to growth, which would cover $1 trillion lost in cuts. Although, as bad as that sounds, it’s actually even worse than that, since the Republican economists also seem to have misstated or misunderstood some of the studies that they claim show the economy would grow as much as they say.

It seems like the point of this is more about coming up with a case that these tax cuts would recoup much of their costs than it is about coming up with a convincing case  that they would — without, of course, explicitly saying so. Better to let the politicians make that leap. But in any case, the fact that their estimates are so far outside what Holtz-Eakin himself has said about tax cuts you could expect that “at best” tells you how seriously you should take the Senate plan.

We shouldn’t be having this debate again. We know that tax cuts don’t pay for themselves. Ronald Reagan’s didn’t in 1981, George W. Bush’s didn’t in 2001, and there’s no reason to think Donald Trump’s would in 2017. Not when our best estimates are that it would barely increase growth. Indeed, only one of the 38 top economists in the University of Chicago’s ideologically diverse poll agreed that the Republican tax plan would make the economy “substantially” bigger.

And yet here we are playing the same game as always: Republican politicians are trying to convince themselves that deficit-financed tax cuts would not be fiscally irresponsible, and Republican economists are trying to come up with reasons that might be right.

These are the same people who were telling us the debt was an existential threat a few years ago. I wonder what changed.