For President Biden, the outcome of the midterm elections will be critical, both for the fate of his policy agenda and for his ability to function without partisan distractions for the next two years.

The president has already struggled to pass legislation he promised as a candidate because Democrats hold a bare majority in the House and the ability to break a 50-50 tie in the Senate. But that difficulty will become a near-impossibility if Republicans take control of the House or the Senate — or both. Republicans would not only have enough votes to defeat most of Mr. Biden’s proposed legislation, but they would be able to keep Democratic measures from even being considered.

Furthermore, Republican control of Congress would put Mr. Biden’s political enemies in charge of investigative and oversight committees. Republicans have already vowed to use those positions to conduct high-profile inquiries into Hunter Biden, the president’s son; the administration’s handling of migrants at the border; and the chaotic exit of U.S. troops from Afghanistan.

The result would most likely be a White House in a defensive crouch, constantly forced to respond to demands for information from congressional committees. Some White House staff would hire lawyers to defend themselves against subpoenas requested by Republican-led investigations. And White House briefings would be filled with questions about the newly empowered Republican majority.

Previous presidents have faced the same situation. President George W. Bush called the 2006 midterms a “thumping” after Democrats won control of both chambers. In 2010, Republicans won back the House in what President Barack Obama called a “shellacking” by his adversaries. In both cases, the shift in the majorities hampered the presidents’ agendas and ratcheted up the partisan attacks from Capitol Hill.

— Michael D. Shear