President Trump, Vice President Gore, Speaker Mike Johnson, and Congresswoman Alexandria Ocasio-Cortez all have one thing in common — they are all invested in Tennessee’s seventh congressional district, which will elect a new representative on Tuesday. The deep-red district, which Mr. Trump won by more than 20 points last year, may go to the Democrats in what could be a shocking update. 

The district — which encompasses the suburbs and rural areas to the west and southwest of Nashville — should go to Republicans easily. But Democratic nominee Aftyn Behn’s campaign has capitalized on the rising cost of living, her party’s anger at Washington, and Mr. Trump’s deep unpopularity in order to make this a single-digit race. 

The previous representative, Congressman Mark Green, resigned from Congress over the summer to pursue a private sector career. This opening represents the first special congressional race this year in a district that is not impossible for a party to flip. According to the most recent polling from Emerson College, Ms. Behn is only two points behind Republican nominee Matt Van Epps — well within the margin of error of four points. 

On Monday, Democrats rallied on a Zoom call with thousands of Ms. Behn’s supporters. Mr. Gore and Ms. Ocasio-Cortez, along with Congresswoman Pramila Jayapal, headlined the event.

“This is an incredibly close race. It’s very, very exciting,” Mr. Gore, who represented Tennessee in Congress for 16 years, told supporters on the call. “I have never seen the political tides shift as far and as fast as we’re seeing them move in this election.”

Mr. Gore said the cost of groceries and rising health care premiums are likely a driver behind Ms. Behn’s success. He notes Mr. Trump’s new ballroom being constructed where the East Wing once was as an especially hard image for the GOP to defend. 

“This is by far the most corrupt administration in history,” the former vice president said. “What are Republicans in Congress doing in response to this reckless and dangerous behavior? They’re enabling it.”

Ms. Ocasio-Cortez says she spoke by phone with Ms. Behn in recent weeks, which spurred her to support the Democratic nominee’s campaign. 

“I genuinely believe that we are at a time where people are increasingly recognizing that our fights are not left and right, but they are top and bottom,” Ms. Ocasio-Cortez told the call’s participants. 

Even though Ms. Behn has been able to turn this deep-red district into competitive territory, she has not done so as a staunch moderate — she has committed herself to being an ardent member of the left flank in Congress. Ms. Jayapal, who previously led the Congressional Progressive Caucus, noted that Ms. Behn is a supporter of universal health care and a repeal of the Republicans’ Medicaid reforms passed over the summer. 

It isn’t just Democrats who are investing their futures in Ms. Behn’s success on Tuesday. Republicans also see this race as an existential one for their party. 

Mr. Johnson, who for the first time this year made a campaign trip for a special congressional election on Monday, rallied Mr. Van Epps’s supporters at several stops along with the held of Mr. Trump. 

Mr. Johnson, wearing a Trump–Vance windbreaker on Monday, appeared alongside several members of the Tennessee congressional delegation, which is almost entirely Republican. 

He went so far as to call Mr. Trump to have him speak to Mr. Van Epps’s supporters. 

“We have to win this seat,” Mr. Trump said via speaker phone during Mr. Johnson’s rally on Monday. “Don’t let [anyone] fool you — the Democrats are spending a fortune.”

The president’s son, Donald Trump Jr., also took time out of his day on Monday on his podcast to urge residents of Tennessee’s seventh district to get out to vote for the Republican candidate. 

“Our superpower as conservatives is to sit it out — to say, ‘We got what we wanted in the big one for president and we’re gonna sit everything else out.’” the younger Mr. Trump said. “You may end up with this lunatic representing you.”

Ms. Behn on the Zoom call on Monday said she and her district are prepared to be “shocking” the country on Tuesday night. 

“Everything comes down to what we do tomorrow,” Ms. Behn told supporters.