Every president of the past 30 years has taken office with his party in control of both the Senate and the House. That combination has allowed presidents to pass major legislation early in their terms — including deficit reduction by Bill Clinton, tax cuts by George W. Bush and Donald Trump, an expansion of health insurance by Barack Obama, and a major climate law by Joe Biden.

The next president may end the streak.

Even if Kamala Harris wins, a Democratic-controlled Senate looks improbable. If Trump wins, his party has a better chance to enjoy a so-called trifecta — controlling the White House, Senate and House of Representatives — but it isn’t assured. “Control of the House is on a knife’s edge,” Maya Miller, who’s been covering the campaign for The Times, told me.

In today’s newsletter, my colleagues and I will preview the race for congressional control. We’ll also explain what a divided government might get done and what each party hopes to accomplish if it does win a trifecta.

The Senate

The Senate math is daunting for Democrats. Their caucus now has 51 senators, meaning they can lose only one seat and retain control in a Harris presidency. (The vice president breaks 50-50 Senate ties.) And Democrats are vulnerable in several states.

West Virginia is all but lost because Joe Manchin is retiring and other Democrats tend to lose badly there. In Montana, Jon Tester, the Democratic incumbent, trailed by eight points in the most recent Times/Siena College poll. A few other Senate Democrats, like Sherrod Brown in Ohio and Tammy Baldwin in Wisconsin, are in tight races.