Editor's Note: (Julian Zelizer, a CNN political analyst, is a professor of history and public affairs at Princeton University and author of the book, "Burning Down the House: Newt Gingrich, the Fall of a Speaker, and the Rise of the New Republican Party." Follow him on Twitter @julianzelizer. The views expressed in this commentary are his own. View more opinion at CNN.)
(CNN) All eyes are on the presidential race. Americans are losing sleep over the high-stakes contest and obsessively tracking poll numbers while spending almost every conversation wondering who will win. The election serves as a referendum on President Donald Trump, and the outcome will influence the direction of public policy, determine the health of our democratic institutions and shape the tenor of our national discourse for years to come.
But even as enormous attention is trained on the presidency, we need to remember that the Senate races will play an equally important role in what happens next.
Senate Majority Leader Mitch McConnell -- with a disciplined Republican caucus backing him -- has been the most important ally to the President since 2017. The dynamic reminds us of the one that emerged after the 1932 election. The great political scientist David Mayhew put it aptly when he wrote that the election provided New York Sen. Robert Wagner with a president, Franklin Delano Roosevelt, who would finally sign his bills.
The only saving grace for Democrats has been control of the House since 2019. If Democrats obtain a Senate majority in 2020, which is a real possibility based on what we are seeing in the polls, this could be a game changer. Democratic control of both the House and Senate would be of great import regardless of who controls the White House.
If former Vice President Joe Biden is elected president, a Democratic Senate would be vital to creating a path toward legislative change. Under Senate Majority Leader Chuck Schumer, Democrats would have the muscle to make progress on one or two priority issues, like shoring up our public health infrastructure or passing an economic stimulus bill that would finally help states, businesses and individuals recover from the economic crisis.
While much of a Biden administration would have to focus on cleaning up the mess created by Trump, there could be room to move forward on issues like climate change or immigration reform that have been ignored for too long. Even if Democrats don't jettison the filibuster, they would be able to use party unanimity and procedures, like the budget reconciliation process, in an aggressive fashion to advance legislation.
Democratic control of the White House and Congress would also create an opportunity to tackle structural issues, such as the size of the Supreme Court. When it comes to federal court nominations, Democrats could also start the slow process of reversing the conservative shift that took place under Presidents Bush and Trump.
If Republicans remain in control of the Senate, however, Biden will be hamstrung. Even with a devastating outcome for the Trump campaign, Senate Republicans would likely double down on obstruction. As we saw after Republican presidential nominee John McCain's defeat in 2008, Republicans only grew more radical under former President Barack Obama. The same dynamics would be at play.
In this scenario, a President Biden would find himself gridlocked while Republicans would try to rebuild their forces for the midterm in an attempt to gain even more Congressional power. Republican obstruction would also aggravate internal tensions within the Democratic Party that have been buried under the opposition against Trump. In other words, President Biden needs a Democratic Senate and one with a decent-sized majority.
If President Trump wins reelection, a Democratic Senate would be the only way to check an administration intent on moving at a furious pace to achieve its remaining goals — from more supply-side tax cuts to draconian immigration restrictions. While the 2018 midterms allowed the Democratic majority in the House to throw sand in the gears of the Trump administration's plans, winning control of the Senate in 2020 would give Democrats considerably more power to check the President. It could force a debate about holding the President accountable for corruption, abuse of power and conflict of interest in a way that was previously impossible. Impeachment -- and conviction, this time around -- would certainly be on the table in this case.
And if things stay the same, with Republicans retaining control of the Senate and President Trump winning reelection, watch out. Democrats will find themselves totally marginalized with an emboldened GOP trying to go for broke on every piece of the party's "dream legislation," placing immense pressure on House Democrats from moderate districts to go along with some of their ideas.
In this scenario, we would see a completely unrestrained President, and a rightward turn so dramatic that the nation might veer off a cliff.
Given the stakes, all eyes should be on the Senate races. Whether Democrats can pick up seats in states like Colorado, Maine, Georgia and North Carolina while holding onto the others will be as much of a game-changer as what happens in the Electoral College. Though many Americans don't tend to follow the ins-and-outs of Capitol Hill with the same intensity as the White House, American history shows us that what happens in Congress often determines what happens in Washington.