As of yesterday, the answer to that question was obvious: She didn't want to give a single iota of leverage or momentum to Republican Senators looking for an escape hatch on a tough vote. President Trump has been pushing GOP members hard to oppose the House-passed bill that would undo his national emergency declaration, so a Democratic Speaker wasn't about to give wavering Senators any political cover by contemplating a plausible Plan B. Within that context, this ultimatum makes sense:
In an effort to avoid voting in favor of the House's resolution to terminate Trump’s #FakeEmergency, GOP senators are proposing legislation to allow Trump to violate the Constitution *just this once.*— Nancy Pelosi (@SpeakerPelosi) March 13, 2019
The House will not take up this legislation to give President Trump a pass.
But once today's expected Senate vote is in the rearview mirror, and the resolution is (presumably) passed, might she change her tune? It's quite clear that neither house of Congress can garner two-thirds majorities to override the coming presidential veto -- so if a bipartisan appetite truly exists to limit 'emergency' authority, why not take up the baton, post-veto? Utah Senator Mike Lee introduced a piece of legislation that would rein in the sprawling powers Congress has handed to the executive on this front over the years, and Republican Senators have reportedly grown "mostly united" behind the sensible plan's concept:
Under a 1976 law, presidents have wide discretion in determining when a national emergency has occurred. Congress can vote to block an emergency declaration, but the two-thirds majorities required to overcome presidential vetoes make it hard for lawmakers to prevail. Lee's proposal says a presidential emergency would last 30 days unless Congress votes to extend it. It would apply to future emergencies, but would not Trump's current border emergency unless it is refiled next year, as is required annually by law to be continued. A vote on Lee's plan was expected after Congress returns from a recess later this month.
Even many of the Republicans who've defended President Trump's border emergency decree (my objection has been narrow but firm) have argued that the legislative branch has surrendered too much power to the executive branch on this issue. Trump's action is, perhaps regrettably, within the bounds of the law, they've sighed. If that's how they really feel, they should be enthusiastically on board for a measure that claws back some of said power. Relatedly, if a Trump veto kills the emergency declaration-cancelling resolution, as is almost certainly going to the case, shouldn't Democrats be eager to join their GOP colleagues in formally shifting the 'emergency' dynamic back toward Congress? Put another way, once the immediate partisan utility of opposing Lee's bill expires, why wouldn't Pelosi bring the proposal up for a vote?
I'd guess there's a decent chance that she might do exactly that. But there's also a possibility that Democrats aren't genuinely interested in handcuffing a future Democratic president's powers, even if that means restricting Trump's options for the remainder of his term in office. While I would have voted with unified Democrats in favor of the anti-emergency resolution, I haven't forgotten that Nancy Pelosi and friends routinely cheered President Obama's egregious unilateral abuses because they supported that power abuser, and the ends he sought to achieve. Once we've reached the conclusion of this current battle, I'd love to hear Democrats' potential reasons for opposing Lee's legislation. But wait, how will Republicans react to Trump's veto threat?
That might be a death knell for Lee's bill. The White House seemed to be open to this plan as a compromise that could provide a fig leaf for GOP Senators to back the president today; the idea was that legislators could argue that POTUS shouldn't have so much power in this arena, even though he does -- so they'll vote 'no' on the resolution, then 'yes' on Lee's bill to rectify the problem moving forward. But if Trump won't sign Lee's bill no matter what (presidents are loath to give up power), Mitch McConnell would probably be inclined to deep-six the proposal entirely, despite having told reporters that he "may well" support it. Which brings us to an ironic possibility: If and when the House-passed 'emergency' resolution dies on Trump's desk, might Pelosi's smartest chess move be to...immediately take up the very bill she rejected yesterday? I'll leave you with National Review's house editorial urging Republicans to break with the president on today's resolution vote: