Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderGraham: GOP has votes to confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda MORE (R-Tenn.) on Thursday urged President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE to reconsider his decision to declare a national emergency to construct the U.S.-Mexico border wall, warning that Trump was creating a "constitutional crisis."

"I want to make a respectful suggestion and that is that President Trump ask his lawyers to take a second look at existing funding authority that the president has," Alexander said during a Senate floor speech.

Alexander, who is retiring at the end of this Congress, outlined several concerns that he has about Trump's decision to declare a national emergency, but did not specifically say if he would vote for or against a resolution to block it, which is expected to get a Senate vote in the next few weeks.

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Echoing concerns voiced by several Republicans, Alexander noted that Trump could set a precedent for a future Democratic president to leapfrog Congress and enforce policies that Republicans would oppose, including on gun violence or climate change.

"There is no limit, the imagination of what the next left-wing president could do to harm our country with this precedent," he said.

Alexander also portrayed Trump's decision as contradictory to the system of government set up by the Founding Fathers, noting that Congress has the responsibility for appropriating money.

"This is what Justice [Antonin] Scalia said [about separation of powers]. 'Every tin-horned dictator,' the justice said, 'every tin-horned dictator in the world today has a bill of rights. That's not what makes us free. What has made us free is our Constitution,'" he said.

Pressed by reporters after his speech if he would vote for a resolution of disapproval if Trump doesn't reverse course, Alexander sidestepped the question. He added that he gave both the White House and Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.) a copy of his floor speech in advance.

"I hope that the White House will interpret my message as a respectful request to the president to take another look at the authority he already has, which would let Republican senators who want support him on border security to be able to do that," he said.

Trump announced earlier this month that he would declare a national emergency after Congress passed a funding bill that included $1.375 billion for physical barriers along the border — significantly less than the $5.7 billion requested by Trump.

Including the $1.375 billion passed by Congress, Trump is hoping to use a combination of executive actions and the emergency declaration to pull together roughly $8 billion for the border.

Alexander, during his floor speech, outlined an alternative way that Trump could get $5.7 billion without declaring a national emergency. He pitched the president on moving an additional $1.5 billion out of the Pentagon's counterdrug activity accounts, which would allow him to build the 234 miles of border barriers that he previously requested.

"Using funds already approved by Congress avoids the constitutional crisis of separation of powers," Alexander said.

Alexander's speech comes as the Senate is expected to vote on a resolution blocking Trump's national emergency declaration during the next two weeks.

Three GOP senators — Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (Maine), Thom Tillis Thomas (Thom) Roland TillisThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting OVERNIGHT ENERGY: EPA head questions connection of climate change to natural disasters | Pebble Mine executives eye future expansion in recorded conversations | EPA questions science linking widely used pesticide to brain damage in children Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight MORE (N.C.) and Lisa Murkowski Lisa Ann MurkowskiSenate GOP aims to confirm Trump court pick by Oct. 29: report Senate GOP sees early Supreme Court vote as political booster shot Pence defends Trump's 'obligation' to nominate new Supreme Court justice MORE (Alaska) — have said they will support the resolution. With every Democrat also expected to vote for it, Democrats are one vote short of being able to block Trump's emergency declaration and force Trump to use a veto.

Several Republicans, including Alexander, have voiced concerns about Trump's decision to declare a national emergency and have yet to say how they will vote.

The House on Tuesday passed its own resolution seeking to block Trump’s declaration.