JACKSON — At the Neshoba County Fair in August 2018, U.S. Sen. Roger Wicker sought to assure a suspicious, conservative crowd of his allegiance to President Donald Trump's agenda.

Some in the crowd booed and waved Mississippi state flags at the white-haired Republican who, three years earlier, had drawn ire after he called for changing that flag to rid it of Confederate imagery. Instead of retreating from that view, Wicker spent his 2018 re-election campaign touting his pro-Trump voting record.

On Thursday, though, Mississippi's senior senator defied Trump on the president's most central campaign promise when he voted against Trump's national emergency declaration to build a border wall along the U.S.-Mexico border. He joined 11 other Republicans and every Democratic senator to pass a resolution blocking the move.

Moments after the resolution passed, Trump sent out a one word tweet: "VETO!" he wrote.

In a statement Thursday, Wicker said he had held "very cordial conversations with the president" ahead of the vote and "strongly supports" building a wall, but believes "an emergency declaration was the wrong approach."

Trump's declaration raised concerns that it violates the Constitution's separation of powers, which grants Congress the power to appropriate funds—not the president. Trump's declaration would take funds that Congress appropriated for other purposes and use them to construct a wall the Democratic-controlled body did not approve.

Since Congress passed it in 1976, the National Emergency Act has typically been used in response to disease epidemics like swine flu or natural disasters like wildfires, earthquakes or hurricanes. A president has never used it to bypass a Congress that denied him funding due to a policy disagreement.

"I am concerned about the precedent an emergency declaration sets, which might empower a future liberal president to declare emergencies to enact gun control or to address 'climate emergencies,' or even to tear down the wall we are building today," Wicker said in Thursday's statement.

That argument echoed one many Democrats have also made. Last month, California Sen. Kamala Harris, a 2020 Democratic presidential candidate, called Trump's wall a "vanity project," and listed what she called "real national emergencies," including climate change and gun violence. If allowed to go forward, Trump's declaration could set a precedent that could come back to haunt Republicans.

"I regret that we were not able to find a solution that would have averted a challenge to the balance of power as defined by the Constitution," Wicker said in his statement. "The system of checks and balances established by the Founders has preserved our democracy. It is essential that we protect this balance even when it is frustrating or inconvenient."

During the 2018 campaign, Wicker would often point the data analytics gurus at FiveThirtyEight, who found he had voted for Trump's agenda about 96 percent of the time. It had already dropped some since the election, but after Wicker's vote today, his pro-Trump score in the newly elected Congress fell to 80 percent.

Mississippi's other U.S. senator, Republican Sen. Cindy Hyde-Smith, voted against the measure. Like Wicker, she spent the 2018 election highlighting her loyalty to Trump. There is a difference, though: While Wicker will not face re-election until 2024, Hyde-Smith must run for re-election next year. Her 2018 race was to finish out the final two years of former Sen. Thad Cochran's term. He retired last March for health reasons.

In a statement after the vote Thursday, Hyde-Smith said Trump was "justified" in his use of the national emergency powers.

"An emergency declaration may not be an ideal course of action, but an objective look at surging unlawful border crossings and illegal drug trafficking indicates we are facing a crisis that will get worse before it gets better," she said. "This is a serious issue. The citizens of this country would be better served if Congress worked together to address this humanitarian and border security crisis, rather than using the issue to score political points."

The U.S. Department of Homeland Security did report a surge in the number of undocumented immigrants entering the country illegally in February. However, much of that can be attributed to the administration's policies.

In the past, refugees families could apply for asylum at ports of entry. Late last year, the administration shut down numerous ports, leaving asylum seekers with fewer options. At those that remain open, they are put on waiting lists, a practice known as "metering." As a result, more refugee families are crossing the border and turning themselves into border agents to apply for asylum. DHS is reporting those families as having entered illegally.

In a report last year, the department itself admitted that "metering" causes a rise in illegal border crossings.

"According to one Border Patrol supervisor, the Border Patrol sees an increase in illegal entries when aliens are metered at ports of entry. Two aliens recently apprehended by the Border Patrol corroborated this observation, reporting to the OIG team that they crossed the border illegally after initially being turned away at ports of entry," the report said.

Overall, though, illegal border crossings have fallen to historic lows in recent years. In 2017, border agents apprehended 303,916 people entering the border illegally—the lowest number since 1971. For comparison, back in 2000, agents apprehended nearly 1.7 million illegal entrants.

Hyde-Smith's pro-Trump ranking at FiveThirtyEight is now 90 percent, down from a perfect 100 percent pre-election. It took a hit after she voted on Feb. 4 to disapprove of Trump's planned withdrawal of U.S. troops from Afghanistan and Syria. Wicker also voted against Trump on that measure.

If the president does veto the measure as he has promised, the Senate will be shy of the two-thirds majority it would need to override it.

In the House, all three of Mississippi's Republican representatives voted with Trump and against the resolution, including Reps. Steven Palazzo, Michael Guest and Trent Kelly. Mississippi's lone Democratic congressman, Rep. Bennie Thompson, voted for the resolution.

Thompson chairs the U.S. House committee that oversees DHS, and has repeatedly criticized the department's treatment of the asylum seekers at the border, including two immigrant children who died in border patrol custody in December.

Follow state reporter Ashton Pittman on Twitter @ashtonpittman. Email story tips to [email protected].