A month ago, the Chicago convention felt like an Irish wake. Now, as Democrats gather for their first in-person convention since 2016, it feels like an epic party.

This week’s DNC will be a rollicking celebration of a united and energetic party that is leading in the polls and has the most valuable thing a campaign can have: momentum.

Vice President Kamala Harris will take the stage Thursday night having defied all rules of politics and gravity. Over the last month, she has reversed the death spiral of polls that had predicted almost certain defeat for Democrats on every ballot, and is now ahead nationally and in key swing states. She has traveled the country, drawing crowds reminiscent of the “Yes We Can” days of the 2008 Obama campaign, raising record amounts of money and opening new paths to the 270 Electoral College votes needed to win. All while putting Donald Trump and the Republican Party on the defensive, seemingly lost and without a message.

But Democrats should not become arrogant or comfortable. In politics, things can and often do change, and quickly. Just ask former President Donald Trump.

When Donald Trump left the stage of his convention in Milwaukee just over a month ago, he seemed well on his way to a second term. It was the Republicans who were united and energized. It was the Republicans – fresh off the assassination attempt and Biden’s disastrous performance in the debate – who had the momentum. And it was Donald Trump who led in every national and swing state poll.

Once the balloons hit the floor of the United Center on Thursday night, Kamala Harris and her running mate, Gov. Tim Walz of Minnesota, will likely enjoy a “convention surge” and extend their leads nationally and in swing states. But Democrats would be wise to heed the words of their newly-minted vice presidential nominee and former high school football coach, who recently said, “It’s only halftime and we’re down by a touchdown.”

Never has our political body suffered such a shock as it has in the last month. And how it will all unfold in the few remaining weeks of the 2024 election campaign is yet to be seen. But a look back at the last two election cycles should sober Democrats as they leave Chicago after four days of partying.

The last time Democrats met in person for a convention, Hillary Clinton left the convention in Philadelphia with an even bigger poll lead than Harris does today. That lead continued to grow in the fall, and two weeks before Election Day, Clinton was leading Trump by an average of 7 percent. And every Democrat remembers how that turned out.

And if looking back eight years is either too long ago or too traumatic for the Democrats, they only need to look back four years. Two weeks before election day, Joe Biden was ahead of Trump by an average of nine percent. Biden would win, of course. But only by four percent and with only a narrow lead in the crucial swing states, which would be needed to get him over 270 in the Electoral College.

The last two election cycles have underscored two truths that Democrats should keep in mind as we head toward the end of this historic race: The country is fairly evenly divided, and Trump has proven, at least in the past, to be an effective political campaigner.

When you also consider the Republicans’ fairly large structural advantage in the Electoral College, Tim Walz’s advice to Democrats to “play like we’re a touchdown behind” seems particularly wise.

None of this is to diminish the herculean task that Kamala Harris has accomplished almost perfectly over the last month. Nor is it to suggest that Democrats are wrong to celebrate this week in Chicago. With early voting starting in less than a month, almost every political scientist in the world would say they would rather be Democrat than Republican in the home stretch.

But the Democrats should not rest on their laurels. The election will be close. And it will probably be very close.

And as the last month has shown us, may and probably will happen.

Doug Gordon is a Democratic strategist and co-founder of UpShift strategies who has worked on numerous campaigns at the federal, state, and local levels, as well as on Capitol Hill. On Twitter/X he is @dgordon52.

The views expressed in this article are those of the author.