Another week and there is more trouble ahead for President Donald Trump in his pursuit of funding for a border wall with Mexico.

Trump and Senate Republican leaders are struggling to keep fellow GOP senators in line behind Trump's declaration of a national emergency along the U.S.-Mexican border. But GOP support is eroding. The latest dissenter is Sen. Rand Paul of Kentucky, who said Saturday night that he couldn't back Trump's position because the president was exceeding his constitutional authority and improperly limiting the role of Congress in approving federal expenditures. "If we take away those checks and balances, it 's a dangerous thing," Paul said at a Kentucky Republican party dinner, according to the Bowling Green Daily News.

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On Sunday, Paul added to his explanation in an essay on the Fox News web site. "I stand with the president often, and I do so with a loud voice," Paul wrote. "Today, I think he's wrong, not on policy, but in seeking to expand the powers of the presidency beyond their constitutional limits."

The House has already voted to void the emergency declaration as majority Democrats argued that it exceeds the president's authority. The Democrats say there is no existing crisis that would justify such emergency powers allowing Trump to use various government accounts to pay for a wall along the border even though Congress denied him those funds. The promise of a wall paid for by Mexico was Trump's defining issue of his 2016 campiagn.

Republicans hold a over 53-45 majority in the Senate, with two independents who vote with the Democrats. Four GOP senators--Paul along with Susan Collins of Maine, Lisa Murkowski of Alaska, and Thom Tillis of North Carolina--have said they plan to vote against Trump's emergency declaration. If all the Democrats and the independents oppose Trump as expected, he would lose the vote on the emergency. But then he could veto the legislation, and it's unlikely that his opponents could muster the two-thirds super-majority in the House and Senate that would be needed to override his veto.