While unprecedented, the tactic could theoretically manipulate the Constitution’s funding laws to successfully bypass Congress and allow a sitting president to reallocate funds without congressional permission. Even though the House passed the resolution to terminate the declaration of emergency, Trump has vowed to veto the resolution if it should make it to his desk. So how can Congress overrule a veto, and how rarely is it done?

When a president vetoes a bill, Congress can only override the veto by taking a second vote in both chambers and passing the bill with a two-third majority in both houses. Historically, fewer than 10% of all presidential vetoes have been overturned, or 106 in total. The last time Congress over-ruled a veto was October 11, 2000, when Bill Clinton’s bill Energy and Water Development Appropriations.

The rarity of a veto override is attributed to the bipartisan conflict of each chamber of Congress. For example, in 2000, the Republicans held a majority in both the Senate and the house when they overruled the sitting Democrat president. The current Congress is divided between a Democrat-lead House (235-199) and a Republican-led Senate (53-45). Achieving a two-thirds majority in both chambers of Congress is simply unlikely when they are led by separate parties.

As for the resolution to overrule Trump’s national emergency, the Senate is set to vote on the resolution before March 18. Since it’s a privileged measure, no filibustering is allowed; only a majority will pass or defeat the resolution. The crux of the matter is whether enough Republican Senators can be swayed to vote with the Democrats. As of Friday, three Republican Senators have vowed to to do so: Susan Collins of Maine, Lisa Murkowski of Alaska, and Thom Tillis of North Carolina. Only four more would be needed to send the resolution to Trump, assuming all Democrat Senator voted with their party. According to Five Thirty Eight, if Trump vetoes the measure, then both chambers of Congress are short of the votes needed to override (50 short in the House and 20 short in the Senate).

Thom Tillis wrote an op-ed for The Washington Post explaining his position to vote against party: “As a U.S. senator, I cannot justify providing the executive with more ways to bypass Congress,” Tillis explained. “As a conservative, I cannot endorse a precedent that I know future left-wing presidents will exploit to advance radical policies that will erode economic and individual freedoms.”

Prior to the House vote, Speaker Nancy Pelosi addressed House Republicans, “Is your oath of office to Donald Trump or is it to the Constitution of the United States? You cannot let him undermine your pledge to the Constitution.”

Meg Hanson is a Brooklyn-based writer, teacher, and jaywalker. Find Meg at her website and on Twitter @megsoyung.