U.S. Sen. Lamar Alexander doubled down Thursday on his condemnation of the President Donald Trump's declaration of a national emergency to fund a border wall, calling it a "constitutional crisis."

During a speech on the Senate floor, Alexander offered an alternative for border funding that would not set a "dangerous precedent."

Tennessee's senior senator once again called Trump's declaration of an emergency as "unwise" and "inconsistent with the U.S. Constitution."

"I support what he wants to do but do not support the way he's been advised to do it," Alexander said, saying he'd like Trump to ask his lawyers to take a second look at existing funds for a border wall.

Alexander voted this month in favor of providing $22 billion for border security, including 5,000 more detention beds, funding for additional border agents and security at entry points.

It also included $1.37 billion for 55 more miles of physical barrier or wall along the border with Mexico — far less than the $5.7 billion the president had initially demanded.

Trump declared a national emergency on Feb. 14 as a strategy to secure $5.7 billion in funding for 230 miles of a steel pole wall along the southern U.S. border.

The emergency declaration is expected to create an extended legal and political battle. Trump himself predicted the issue would end up before the U.S. Supreme Court.

"There has never been an instance where a president of the United States has asked for funding, Congress refused it and the president then has used the National Emergency Act to justify the funding anyway," Alexander said.

"There is no limit to the imagination of the next-left wing president could do to harm this country."

Alexander offered an alternative to fund Trump's wall:

$1.37 billion from the Homeland Security Appropriations Bill;

$601 million from Treasury Forfeiture fund;

$3.7 billion from the Department of Defense counter drug enforcement fund.

"(Trump) can avoid this constitutional crisis ... and use the funds already approved by Congress to avoid establishing a dangerous precedent," Alexander said.

Alexander asserted that using already approved funds would avoid "months and years" of litigation that would make it "very unlikely" that a border wall is ever built.

The route would allow the Senate to both support the president's wall while also supporting the Constitution, he said.

Alexander said in a statement earlier this month that while Trump can declare a national emergency to build a wall, the next president can declare a national emergency to tear it down.

In the statement he added a president could similarly "declare a climate change emergency" that would result in the closure of coal plants or, by calling a "health care emergency," force Americans out of private insurance and onto Medicare.

Reach Yihyun Jeong at yjeong@tennessean.com. Follow her on Twitter @yihyun_jeong.