Rand Paul will vote for bill overturning Trump's national emergency declaration. Here's why.

Despite supporting President Donald Trump for most of his presidency, Sen. Rand Paul (R-Ky.) revealed over the weekend that he will not support Trump's national emergency declaration.

Trump issued the declaration last month as a last-step measure both to prevent another partial government shutdown and to obtain funding for a wall on the U.S.-Mexico border, which Trump believes will alleviate an alleged illegal immigration crisis.

Why does Paul oppose Trump's declaration?

During an event in Kentucky over the weekend, Paul explained that he will vote to support a resolution that seeks to overturn Trump's declaration.

"I can't vote to give the president the power to spend money that hasn't been appropriated by Congress," Paul said, 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."

For Paul, supporting Trump's declaration while opposing former President Barack Obama's sweeping executive orders would be a capitulation leading to the loss of his "political soul," he explained in an op-ed for Fox News.

"Every single Republican I know decried President Obama's use of executive power to legislate. We were right then," Paul wrote. "But the only way to be an honest officeholder is to stand up for the same principles no matter who is in power."

The Kentucky lawmaker, who traditionally has strong libertarian leanings, said he must stand on his principles, giving his allegiance to the Constitution first, even if that means blocking political ambitions he supports.

"My oath is to the Constitution, not to any man or political party. I stand with the president often, and I do so with a loud voice. Today, I think he's wrong, not on policy, but in seeking to expand the powers of the presidency beyond their constitutional limits," Paul wrote.

"I understand his frustration. Dealing with Congress can be pretty difficult sometimes. But Congress appropriates money, and his only constitutional recourse, if he does not like the amount they appropriate, is to veto the bill," he explained.

What is the significance of Paul's decision?

Paul becomes the fourth Republican senator to pledge to vote against the resolution when it comes to the Senate for a vote later this month. Since the House already passed the resolution, with 13 Republicans joining a Democratic majority, Paul's deciding vote will likely set up the first veto of Trump's presidency.

Republican Sens. Thom Tillis (N.C.), Susan Collins (Maine), and Lisa Murkowski (Alaska) have also pledged to support the resolution.