Analysis by Stephen Collinson, CNN

(CNN) -- President Donald Trump's allies are mounting a stiff defense of his declaration of a national emergency amid increasing signs that the biggest threat to his border wall now comes not from Congress, but the courts.

One of the President's top policy advisers, Stephen Miller, on Sunday indicated that Trump would cast the first veto of his presidency to defend the highly contentious declaration if lawmakers seek to terminate it.

Democrats argued that Trump had launched an unconstitutional power grab because Congress refused to grant his request for billions of dollars in wall funding, and he decided to fund it anyway with money appropriated for other purposes including military construction projects.

And California's Attorney General Xavier Becerra promised to "definitely and imminently" launch a legal challenge to the emergency declaration, one of an expected flurry of suits against Trump's aggressive executive power move.

Trump's announcement, made during a rambling news conference Friday, came after he admitted defeat in his bid to get Congress to pay $5.7 billion in wall funding, but avoided triggering a second government shutdown.

During that event, the President appeared to undermine his own arguments in several ways by saying he "didn't need" to declare the emergency but wanted to go faster than Congress on border security.

Miller tried to clean up that remark in an appearance on "Fox News Sunday."

"What the President was saying is, is that unlike past presidents, he could choose to ignore this crisis, choose to ignore this emergency as others have, but that's not what he's going to do," Miller said.

The senior adviser, one of the most hardline voices on immigration in the White House, also made clear that if Democratic House Speaker Nancy Pelosi initiates an effort to wipe out the declaration, using a clause in the National Emergencies Act of 1976, Trump would respond.

"He's going to protect his national emergency declaration, guaranteed," Miller said.

Congress should 'take back power'

Once the House acts, Senate Majority leader Mitch McConnell is unlikely to be able to prevent a termination effort from coming to the floor, and given skepticism among some Republicans, it's possible a majority could be assembled against Trump's gambit even in the Senate.

Republican Rep. Will Hurd, who represents a vast district along the southern border in Texas, said he would back a measure in the House that prevents Trump taking money away from military construction projects for the wall.

"I'm always open to making sure that Congress takes back some of this power as a co-equal branch of government. And I'm sure there's going to be a lot of conversations," Hurd said Sunday on "Face the Nation" on CBS.

But one of the President's closest congressional allies, Rep. Jim Jordan of Ohio, predicted that Trump's critics would not be able to put together a two-thirds majority in each chamber of Congress to override any presidential veto.

"I think there are plenty of votes in the House to make sure that there is no override of the President's veto. So it's going to be settled in court," Jordan said Sunday on ABC News "This Week."

Republican Sen. Ron Johnson expressed concern about Trump's claim of authority to act on the wall — which some of his colleagues have warned could establish a precedent for a future Democratic president to take aggressive executive action on an issue like climate change or gun control.

"I wish he wouldn't use it in this case," Johnson told NBC's "Meet the Press" Sunday and warned that a "diminished" Congress had ceded power to the presidency and the courts.

But the Wisconsin senator also appeared to offer himself an out from voting against Trump's assertion of power, suggesting there is a good chance that the GOP will, as is often the case, decline to constrain its president.

"We're going to take a very careful look at what he's doing here in this instance," Johnson said. "But, again, I have to stress this president's been thwarted in his attempt to keep this nation safe and secure. To secure our borders.

"Let's face it, if this president can claim a mandate on anything he ran on, it's exactly this issue: better barriers and securing our border."

While polls show a majority of Americans oppose the wall and don't support a declaration of national emergency to fund it, a CNN/SSRS survey released this month showed that 64% of Republicans do support a national emergency to get a wall and 72% of conservative GOPers back it.

Hard to image a 'poorer case'

Democrats sought to portray Trump's action as a gross abuse of power, arguing that it differs from emergency declarations by previous presidents and could reshape the relationship between the executive and Congress.

"This is the first time a President has tried to declare an emergency when Congress explicitly rejected funding for the particular project that the president is advocating," House Democratic Intelligence Committee Chairman Adam Schiff said on CNN's "State of the Union" Sunday.

"If we surrender the power of the purse, which is our most important power, there will be little check and no balance left. It will not be a separation of powers anymore, just a separation of parties. So this is going to be a moment of truth for my GOP colleagues."

Another Democrat, Hawaii Sen. Brian Schatz, resorted to humor to mock Trump's use of executive power.

"Day 3 of Emergency. Checking in from Oahu. Light rain, 66 degrees. Be careful out there," Schatz wrote on Twitter on Sunday.

On a more serious note, Becerra said that the California court challenge would use Trump's own remarks to undermine his declaration and would argue that he has created an artificial emergency that does not meet historic standards of such declarations.

"We are prepared, we knew something like this might happen. And with our sister state partners, we are ready to go," said Becerra on ABC's "This Week."

"There is enough evidence to show that this is not the 9/11 crisis that we faced back in 2001, it's not the Iran hostage crisis we faced in 1979," Becerra said.

"It's not even the type of national emergency where we are trying to take action against a foreign enemy or to avoid some type of harm befalling Americans abroad."

Trump is unrepentant. An emergency declaration in many ways is a neat political device that allows him to move on from his defeats in Congress over the wall. Even if his declaration is blocked by the courts, it will give him a crusade with which he can rally his base in his 2020 re-election fight.

In a sing-song refrain, Trump said Friday that he believed his effort would be quickly rejected in court but seemed to pin his hopes on an eventual favorable ruling in the Supreme Court with its new conservative majority.

Until then, Trump is insisting, without evidence, that he is getting on with fulfilling the campaign promise that lit a fire under his 2016 campaign — and which he seems to be betting could deliver him a second term.

"BUILDING THE WALL!" the President tweeted out of the blue on Saturday during his long weekend at his private Florida resort.