Keller @ Large: How The Southern Border Wall Debate Is A Rare Opportunity For Democrats

BOSTON (CBS) -- President Trump's declaration of a national emergency on the southern border - a move entitling him to appropriate funds to spend on the wall that Congress never approved - is facing months of legal challenges. And a Democratic resolution rejecting the presidential declaration is sure to be vetoed if it gets to the White House, a veto that is extremely unlikely to be overturned in the Senate.

So why was Senate Minority Leader Chuck Schumer on the floor Tuesday proclaiming that the battle over the declaration is "a moment in history, a point where things may turn a bit"?

Because Democrats are sensing an opportunity to exploit a rare crack in the wall of Republican unity.

The turn Schumer's talking about is a turning up of the heat on Republicans in Congress to decide: are they co-equal partners in governing? Or folks who simply ask "how high" when the president says "jump"?

House Republicans closed ranks behind the emergency order Tuesday, insisting on its necessity even though the president himself seemed to undercut that case when he said: "I didn't need to do this."

But the Democrats' arguments go deeper than that.

"This is not about the border, it is about the Constitution of the United States," said House Speaker Nancy Pelosi, a widely-repeated claim that the president's move undercuts the balance of power which has gained traction with some Republicans worried about how a future Democratic president might take the same liberties.

Republican Sen. Roy Blunt was led to wonder on national TV: "Is it really an emergency if the President asked the Congress to do it and they failed to? That's different than the way this law has been used in the past."

And Tuesday, Schumer warned his colleagues of a threat to their political bread-and-butter.

"A president doesn't like you for some reason. He says there's an emergency and takes money away from a project in your state that you've worked hard for. That's no way to govern."

So all eyes are on Trump-skeptical senators like Mitt Romney, Rand Paul, and Cory Gardner as they prepare to take up the issue within the next 18 days. Because if they won't stand up to the president on this, perhaps they never will.