Speaker of the House Nancy Pelosi (D-CA) watches as U.S. President Donald Trump delivers his second State of the Union address to a joint session of the U.S. Congress in the House Chamber of the U.S. Capitol on Capitol Hill in Washington, U.S. February 5, 2019.

President Donald Trump can veto a resolution blocking his border emergency declaration. If it clears the Republican-led Senate on Thursday, Democrats lack the votes to override him.

But the episode shows again what the 2018 midterm elections brought the GOP: constant pressure.

Democratic control of the House won't produce much legislation in a divided Washington, but it has already produced a fundamental shift in political leverage.

During the first half of Trump's term, Republicans at both ends of Pennsylvania Avenue protected each other. Congress shielded thepresident from oversight; House and Senate leaders shielded their members from casting politically perilous votes.

The shift in White House accountability has attracted the most attention so far, for good reason. The House Intelligence, Financial Services, Oversight and Judiciary committees have piled overlapping probes of their own onto investigations by prosecutors for special counsel Robert Mueller, the Southern District of New York, the state of New York and the Manhattan district attorney.

Those House initiatives alone could effectively cripple the administration over the next two years. Through the impeachment process, they could even cut it short.

Speaker Nancy Pelosi has signaled the House may ultimately eschew an impeachment battle as divisive and politically counterproductive. But she has the ability to regularly squeeze Trump and his fellow Republicans under any scenario.

Pelosi's gavel