President Donald Trump's legacy is about to be dealt a devastating blow — and it is his own Republican Party that is prepared to deal that to him.

A resolution blocking Trump's declaration of a national emergency, which he proclaimed in order to obtain funding for his border wall, is likely to pass the Senate thanks to a quartet of Republican senators who broke from party lines to support the measure — Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Rand Paul of Kentucky and Thom Thillis of North Carolina.

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"I think there will be 10 Republican 'no' votes. Possibly more," Sen. Paul said Monday. "If (you) look at the history of not only our government, but other governments around the world, governments that begin to be run by emergencies, really bad things can happen."

Speaking at the Southern Kentucky Lincoln Day Dinner, Paul said: “I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress. We may want more money for border security, but Congress didn’t authorize it. If we take away those checks and balances, it’s a dangerous thing."

Trump will likely veto the resolution in order to keep his national emergency in effect. It is unlikely either chamber will be able to muster the need majority to overturn his veto. While this may seem to be good news for the president, it is actually a sign of how his power as president is waning.

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For one thing, the fact that enough Republicans are willing to part ways with the president on his signature campaign issue bodes ill for his presidency.

Although this won't be the first time one of Trump's signature initiatives was undone by Republican defections — his attempt to repeal the Affordable Care Act, and Sen. John McCain's iconic "thumbs down" vote, is a famous example of that — the border wall was the cornerstone of Trump's agenda. It was one of the signature promises of his 2016 campaign and, unlike repealing Obamacare, an issue that was uniquely associated with him rather than the Republican Party in general. By deciding that Trump should not be able to build that wall if doing so requires him to undermine the Constitution, the opposing Senate Republicans have taken an important stand against this specific president and what he represents.

It is also notable that opposition from Republicans and Democrats alike has forced Trump to redefine what he means by a border wall.

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Since he began seriously promoting the idea of a border wall in 2015, the proposed barrier has evolved from a concrete structure that would span the entire southern border to slatted fences that would only cover parts of it. At times the president has suggested constructing the wall using steel; at times, he has even suggested putting solar panels on the wall to provide the country with renewable energy. Now that shutting down the government to get a border wall hasn't worked, the president has had to resort to a constitutionally questionable declaration of a national emergency. Even if his veto of the opposition resolution isn't overridden, which seems likely, he has still forced Republicans to admit that the only way to achieve his most famous goal was to violate the party's own ostensibly cherished principle of small government. And by virtue of the presence of GOP support for the opposition resolution, the Republicans themselves can't deny that this is the case.

Finally, Trump is undermined by the fact that his national emergency will be challenged in court, with a high chance that it will be found unconstitutional.

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"I think we can actually look forward with considerable confidence to the opportunity to look into their own records and so forth, discovery, which might very well come to pass in litigation like this, and examination and cross-examination of witnesses, which might very well come to pass in a case like this. They can be very valuable things in terms of getting to the truth and blowing the rhetoric aside," Sen. Sheldon Whitehouse, D-R.I., told Salon last month. He added, "The question is, can he defend it against court challenges? All he has to do is sign a piece of paper and he can do that, but will the piece of paper be honored and will he exceed his bounds? There are propositions for the court at that point."