WASHINGTON – Donald Trump is entering the breach over presidential powers, and taking fellow Republicans with him.

Trump's late-breaking decision to declare a "national emergency" to help build his border wall comes over the objections of many congressional Republicans, and is perhaps the clearest sign yet of divisions within the GOP as it braces for the 2020 elections.

“I wish he wouldn’t have done it,” said Sen. Chuck Grassley, R-Iowa, echoing comments by GOP colleagues.

Republican misgivings about Trump's order range from the moderate Susan Collins of Maine – "a mistake" – to the more libertarian Rand Paul of Kentucky, who tweeted that "extraconstitutional executive actions are wrong, no matter which party does them."

For his part, Trump said congressional Republicans have moved too slowly on wall funding. Still, he predicted that most will stick with him as things move forward, even as he appeared to blame the Republicans for what he considered a lack of funding for border security.

"People that should've stepped up did not step up," Trump said during a White House announcement Friday.

While Trump acknowledged he will be sued over the emergency declaration – and the courts could block his plan to claw money from other budgets for his wall – Democrats plan to exploit Republican divisions in a political way.

Leaders of the Democratic-run House said they will likely try to pass a resolution of disapproval of Trump's declaration of a national emergency on the border. Congressional rules would force the Republican-led Senate to vote on the measure as well, putting Trump's nominal allies on the record.

"I know the Republicans have some unease about it, no matter what they say," said House Speaker Nancy Pelosi, D-Calif. In a tweet after the president's announcement, Pelosi said she and Senate Democratic leader Chuck Schumer, D-N.Y., "call upon our Republican colleagues to join us to defend the Constitution."

Several Republicans have voiced the concerns Pelosi has talked about: The idea that if Trump can declare a national emergency over a border wall, a future Democratic president can declare a national emergency on any number of his or her priorities, from gun control and climate change to opioids and immigration.

A vote on Trump's declaration would be another sign of how deep Republican divisions might run, and how they may play out between now and the next election in November 2020.

"I don’t think anyone knows," said Scott Jennings, an adviser to Senate Majority Leader Mitch McConnell, R-Ky.

If four Republican senators join all the Democrats, a resolution of disapproval could get through the Congress. Trump would likely veto such legislation, though congressional Democrats could then mount an effort to override the veto, keeping the dispute in the political spotlight.

Beyond Capitol Hill, some Republicans are urging GOP lawmakers to fight Trump. That group includes longtime critics who are contemplating primary runs against the incumbent president in 2020.

"The good news out of this unconstitutional power grab is it will split off some Republicans from this wannabe Franco," tweeted John Weaver, a political strategist for former Ohio governor and possible Trump challenger John Kasich.

Republican angst in Congress seems fueled by the fact that Trump's decision of a national emergency caught many of them by surprise.

Trump and his aides had stopped discussing the national security idea in recent days, after weeks of hearing objections from fellow Republicans. Instead, party members urged the president to sign a new spending bill designed to prevent another partial government shutdown, only three weeks after a record-setting 35-day shutdown that also gave Republicans heartburn.

Trump did not use the words "national emergency" during his high-profile political rally Monday night near the Mexico border in El Paso, Texas. Administration officials like acting White House chief of staff Mick Mulvaney had played down the idea, talking instead about plans to divert money from other budgets to finance the wall without a formal emergency declaration.

Things changed Thursday afternoon, amid worries that Trump might veto a new spending bill because it contained less than a third of the wall money he had demanded.

McConnell, who had urged Trump to forgo the emergency declaration, suddenly took to the Senate floor to announce what sounded like a deal with Trump: He would sign the spending bill to keep the government open, while McConnell would back the declaration.

"He has indicated he is prepared to sign the bill," McConnell said. "He will also be issuing a national emergency declaration at the same time. I've indicated to him that I'm going to support the national emergency declaration."

To be sure, many Republicans back Trump's emergency plan, calling it justified by the need for border security. They also cited Democratic opposition to the wall, noting that many members of the opposition don't want to spend any money at all on any kind of border barrier.

Others point out that the courts, probably the Supreme Court, will make the ultimate decision about whether Trump has the legal authority to take this kind of action.

If the courts block Trump's bid to declare a national emergency, that could potentially reduce Republican tensions.

If the courts back Trump, the nature of presidential relations with Congress will change, even between presidents and their parties.

There have been signs of friction before during the up-and-down relationship between Trump, the maverick businessman who had never run for office before 2016, and his adopted Republican Party.

Congressional Republicans have pushed back on concerns that Trump might try to remove special counsel Robert Mueller over the Russia investigation. They have criticized Trump for his decision to remove troops from Syria and his attacks on NATO.

Last week, hours before Trump delivered his State of the Union address, the Republican-controlled Senate approved legislation that included sharp criticism of Trump over planned withdrawals of U.S. troops from Syria and Afghanistan.

For his part, Trump has blamed party members, particularly former House Speaker Paul Ryan, for their loss of the House during last year's elections, as well as other reversals.

Now come legal and political battles over national emergencies, even after Republicans had asked Trump not to take this step.

Some Republican analysts said the legal and political aspects of a unique situation make it hard to assess long-term impact.

Texas-based political consultant Matt Mackowiak said Pelosi's efforts to force a vote on the resolution "may divide the Republicans somewhat."

After that is anyone's guess.

"This is a constitutional issue and a separation-of-powers issue," he said. "And it doesn’t break down nearly on party lines."

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