Senate Majority Leader Mitch McConnell (R-KY) talks to reporters with Senate Majority Whip John Thune (D-SD) (R) following the weekly Senate Republican policy luncheon at the U.S. Capitol March 05, 2019 in Washington, DC.

The Senate will vote Thursday on whether to block President Donald Trump's national emergency declaration, forcing Republicans to decide if they should check the White House's flex of executive power.

The Democratic-held House has already approved a resolution to terminate Trump's action, which he aims to use to secure $3.6 billion to build his proposed border wall. The GOP-held Senate is expected to pass the measure, as at least four Republicans currently plan to support it.

Those four GOP lawmakers joining with 47 Democrats will give the chamber the majority needed to pass it and send it to Trump's desk. The president has threatened to veto the bill, and both chambers appear not to have the votes to reach the two-thirds majority needed to override Trump.

Trump's declaration has split the Senate Republican caucus, which has to choose whether to back the GOP president's priorities or follow through on a professed skepticism of executive power. On Tuesday, Senate Majority Leader Mitch McConnell told reporters that he thinks "everybody in [his] conference is in favor of the president's position on the wall and on border security," but not necessarily the steps he took to fund the project.

"It is no secret that the use of the national emergency law has generated a good deal of discussion," the Kentucky Republican said. "And we'll continue having those discussions. But it will all come to a head on Thursday. The clock runs, and the vote will occur on Thursday."

McConnell confirmed reports that Republicans have sought ways to amend the National Emergencies Act, the 1976 law the president used to declare an emergency over the southern border last month. He said "we're looking at some ways to revisit the law" as "there's a lot of discomfort" with it.

The Senate GOP leader noted that any changes would not apply to Trump's emergency declaration. McConnell said the question more broadly surrounds whether the 1976 law granted too much power to the president.

Shortly before McConnell spoke, Senate Minority Leader Chuck Schumer urged Senate Republicans to oppose Trump's declaration. He said "there is no factual basis for an emergency," adding that Trump's action "dramatically changes the balance of power in our government."

"We hope our Republican colleagues will have the courage, the courage, to stick up for the Constitution, to stick up for what the Founding Fathers wanted and tell Donald Trump on this one: 'We don't go with you.' Even people who are for the wall should be voting against an emergency," the New York Democrat said.

Trump declared a national emergency last month on the same day he signed a spending bill to fund the government through September. In that legislation, Congress only appropriated $1.4 billion of the $5.7 billion the president sought to build barriers on the U.S.-Mexico border. His demand for the border security funding led to a 35-day partial government shutdown in December and January.

Trump hopes to put $8 billion total toward constructing barriers, including the money allotted by Congress. With the national emergency declaration, he hopes to pull an already approved $3.6 billion from the Department of Defense's military construction account.

He aims to use other executive actions to divert $600 million from the Treasury Department's drug forfeiture fund and $2.5 billion from the Pentagon's drug interdiction program.

Numerous states and groups have filed lawsuits challenging Trump's use of executive power.

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