Analysis by Chris Cillizza, CNN Editor-at-large

(CNN) -- Today is the biggest day of the year for congressional Republicans.

It's the day that their massive tax cut plan is expected to pass the GOP-controlled House and Senate, sending the bill to the other end of Pennsylvania Ave. where President Donald Trump plans to sign it tomorrow.

But, even as Republicans prepare to celebrate what House Speaker Paul Ryan called "a moment decades in the making," there are signs that the GOP is facing real political peril in 2018 tied directly to this tax cut plan.

The new CNN national poll has all sorts of warning signs about the tax bill -- and how the public perceives it. Republicans will argue that people don't know what's in it -- and once they find out they will change their minds.

Maybe! But, at the moment, there's plenty of minds that need changing. Here are seven reasons why, as of today, passing this tax bill looks like a bad gamble for Republicans.

1. Opposition is high -- and growing

A majority (55%) of people oppose the tax legislation as compared to 33% who support it. That's bad! But the trend line is actually worse -- as opposition has grown 10 points since November. And, people of all political persuasions -- Democrats, Independents and Republicans -- all oppose the bill more today than they did last month.

2. People think the plan won't help them...

Roughly 4 in 10 (37%) say the tax bill will make their family worse off than they are right now. Only 1 in 5 (21%) say it will improve their family's current status.

3. ....And will help the Trump family

Nearly two thirds (63%) believe the tax plan will make the Trump family's life better while a meager 5% say the President's family will be worse off as a result of the legislation. "This is going to cost me a fortune," Trump has said of the tax bill -- although independent analysis suggests that's simply not true. And, no one truly knows the exact impact on Trump's finances because he continues to refuse any of his previous tax returns.

4. This is seen as a tax cut for the rich

Two thirds of people in the CNN poll say that the tax plan will do more to benefit the wealthy as compared to just 23% who say it will help the middle class most. While Democrats are nearly united in seeing this plan as a tax cut for the wealthy, more than 1 in 4 Republicans (27%) feel the same way.

Again, peoples' views of who the plan will help most runs directly counter to Trump's assertions about the legislation. Of the middle class, Trump said last week: We want to give you ... a giant tax cut for Christmas and when I say 'giant' I mean giant."

5. Trump is a deeply flawed messenger

At 35% approval in the new CNN poll, Trump is sure to finish his first year in office at a historic low point as compared to other post World War II presidents. While he was never hugely popular -- even while he was winning the White House -- he has seen significant slippage in his numbers over the past year, down from 44% in a January CNN survey.

6. Trump is a deeply flawed messenger -- on taxes

The specifics of Trump on taxes don't look much better for the president and his party. Fifty seven percent of people disapprove of how he has dealt with the issue while just 1 in 3 (34%) approve.

7. People want to see Trump's tax returns

Almost three quarters (73%) of those polled said Trump should release his tax returns, a number that matches where things stood back in January. What's interesting is that almost 4 in 10 Republicans (37%) want Trump to release the returns. Trump has repeatedly refused to disclose anything about his own taxes, insisting that those documents say very little about a person's financial status and noting that he has filed financial disclosure forms already.

Add all of that up and here's what you get: A historically unpopular president trying to sell a piece of legislation that people believe helps him and his wealthy friends and doesn't do much at all for them.

That's a VERY tough sell -- even for someone like Trump who professes to be one of the world's great sales people.

Is it possible that more cash in the average person's pocket will change how they perceive the tax bill? Sure! But, public opinion on major pieces of legislation tends to harden quickly -- and stay hardened. (Look at the lack of movement on the Affordable Care Act for the first 8-ish years of its life!)

Republicans, to their credit, are well aware of the gamble they are taking here. The overwhelming belief within the GOP ranks is that passing a tax cut -- even one that is not terribly popular (or popular at all) at the moment -- is a better gamble than not doing so. The Republican base needs to be fed and watered and Republicans are convinced that this tax plan was their best (only? last?) chance to do just that.

It's a theory! But the numbers in the CNN poll paint a very daunting picture for Republicans planning to make this tax cut the centerpiece of their 2018 campaign to hold control of the House and Senate.

They'll push all their chips into the middle of the table later today. We'll have to wait until next November to see if they go busto.