(CNN) If you knew anything about Donald Trump during the 2016 campaign, it was that he was going to build a wall on our southern border-- and that Mexico was going to pay for it.

Over and over again he said just that; it was consistently his biggest applause line in a stump speech packed with applause lines.

Here's the thing: Mexico was never planning to pay for the wall. No deal had been cut by Trump with Mexican President Enrique Peña Nieto. And now Trump -- backed into a corner by his own rhetoric -- is threatening to shut down the federal government unless the Republican-led Congress authorizes some money for the wall.

The logical impossibility of explaining that position was on full display Thursday when White House press secretary Sarah Huckabee Sanders took questions from reporters for the first time in three weeks.

Let's go through the four times she was asked directly about Mexico paying for the wall and Trump's government shutdown threat. ( A transcript of the full briefing is here .)

1. "The President is committed to making sure this gets done. We know that the wall and other security measures at the border work. We've seen that take place over the last decade, and we're committed to making sure the American people are protected. And we're going to continue to push forward and make sure that the wall gets built."

Sanders doesn't say anything about Mexico -- probably because there is nothing to say. She also makes no mention -- at least in this first answer -- of the shutdown and how far Trump is willing to push it to secure wall funding. "We're going to push forward and make sure that the wall gets built" is saying nothing while trying to appear like you are saying something.

2. "Once again, the President is committed to making sure this happens, and we're going to push forward."

So, in summary: The President is going to make sure this happens. Also, forward-pushing.

3. "I certainly don't think any efforts have been abandoned."

This Sanders response come after a question by CBS' Major Garrett about whether Trump's threat to shut down the government over wall funding means he has given up on making Mexico pay for the wall. Her wording is interesting: No "efforts" have been abandoned. The Trump administration then continues to push to make Mexico pay for the wall? How so? When is the last time Trump or someone in his administration spoke to Peña Nieto about paying for the wall? And what was Mexico's response? Are our efforts paying dividends? Is Mexico more likely to pay for the wall now than they were earlier this year?

4. "Again, this is something the President is committed to. He's committed to protecting American lives. And doing that through the border wall is something that's important. It's a priority, and we're moving forward with it."

Sanders has a story -- and she's sticking to it. Trump is committed to building the wall. Things are moving forward. But, the most interesting part of this question-and-answer comes when the reporter presses Sanders on the fact that Trump has stopped saying publicly that Mexico is going to pay for the wall.

"He hasn't said they're not either," she retorts.

Ahem.

To reiterate: Trump spent two years promising at, literally, every rally that he was going to build a wall and Mexico was going to pay for it. Now suddenly, he is issuing a threat to shut down the government unless Congress allocates money to build the wall. And he's stopped mentioning the whole Mexico-will-pay-for-it thing.

Now that reporters are pointing out that obvious change in rhetoric, the best the White House can muster is the delicious double negative that Trump hasn't not said Mexico is paying for the wall.

OK then!

The simple fact is that no president can make a sovereign foreign government do something they believe to be disadvantageous for them. Trump can bring pressure to bear on Mexico -- particularly in regards trade -- to attempt to extract money for the wall from them. But he just can't make them pay up.

Sanders' answers are part of the White House's ongoing attempt to cloud that reality. Why? Because, for Trump, that promise to make Mexico pay for the wall was so fundamental to his appeal as a candidate that to openly acknowledge it isn't going to happen (and never was) is hugely politically problematic.

But all of the spin in the world can't polish this turd of a campaign promise. Sanders knows that. Trump does too.