WASHINGTON – President Donald Trump has tweeted that "Senate Republicans are not voting on constitutionality or precedent" when a resolution to block his emergency declaration at the U.S.-Mexico border comes to the Senate floor.

But the Republican senators who plan to break ranks and vote with Democrats in favor of the resolution on Thursday say precedent and constitutionality are precisely what they are voting on.

Trump announced he was declaring a national emergency at the border after Congress refused to appropriate the $5.7 billion he wanted for construction of a border wall. Many congressional Republicans have joined Democrats in objecting to the move, which they saw as using emergency powers to bypass Congress’ constitutional authority over spending.

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Five Republicans have said they will vote for the resolution. With all 47 Democratic senators expected to vote for it, their support means the resolution will likely be approved.

Trump repeated his vow to veto the measure in a tweet Thursday. It is unlikely Congress would be able to secure the votes needed to override a veto, which requires the support of two-thirds of the lawmakers in each chamber.

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Here are the Republican senators who have said they plan to vote for the Democratic resolution:

Susan Collins, Maine

Sen. Susan Collins is one of two Republicans who plan to vote for the resolution who are also up for re-election in 2020. The New England moderate has bucked her party on other occasions, such as when she voted with Democrats to block Affordable Care Act repeal efforts in 2017.

She said Trump’s decision to declare a border emergency after Congress denied him funding for a wall, "strikes me as undermining the appropriations process, the will of Congress and of being of dubious constitutionality.''

Mike Lee, Utah

Sen. Mike Lee became the fifth Republican to announce he would vote for the resolution after talks between GOP senators and the White House failed to reach a compromise a day before the scheduled vote.

"We tried to cut a deal, the president didn’t appear interested," Lee told The Hill. "I’ll be voting 'yes.'"

Like the other Republican senators who plan to oppose Trump's move, Lee expressed concern about the precedent set by Trump's declaration.

"Congress has been giving far too much legislative power to the executive branch," Lee told The Associated Press.

Lisa Murkowski, Alaska

The daughter of former Alaska governor and senator Frank Murkowski, Sen. Lisa Murkowski has not been afraid to work independently of her party, even winning a write-in campaign after losing her party’s primary in 2010. She was also the lone Republican to oppose Supreme Court Justice Brett Kavanaugh's confirmation.

"When you use the National Emergencies Act to effectively expand executive powers by legislative acquiescence, I think that sets a dangerous precedent, and I don't think that it's a path that we should take," Murkowski told PBS "Newshour" on Wednesday.

Rand Paul, Kentucky

For the libertarian-leaning Sen. Rand Paul, opposition to Trump's border emergency declaration is in large measure a matter of ideological principle.

"I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress," Paul told a gathering of Kentucky Republicans, according to the Bowling Green Daily News. "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."

Thom Tillis, North Carolina

Sen. Tom Tillis is the other senator up for re-election in 2020 who has said he will vote to block Trump’s emergency declaration.

"As a U.S. senator, I cannot justify providing the executive with more ways to bypass Congress," Tillis wrote in an op-ed in The Washington Post. "As a conservative, I cannot endorse a precedent that I know future left-wing presidents will exploit to advance radical policies that will erode economic and individual freedoms."

Contributing: Deborah Barfield Berry, Michael Collins and John Fritze, USA TODAY; The Associated Press