Democrats looking for a shocking underdog win in a special election in Ohio on Tuesday are hoping not only to snatch a House seat in a historically Republican district, but also to energize their national effort to fight President Trump's agenda and reclaim a congressional majority.

Ohio's 12th Congressional District, in the suburbs of Columbus, has had a Republican representative for 35 years, and has been largely in GOP hands since 1920. But a poll released Monday by Monmouth University showed that Republican state Sen. Troy Balderson, 56, leads upstart challenger Danny O'Connor, 31, by just one percentage point.

Trump, who won the wealthy and highly educated district by double-digits in 2016, went all-in for Balderson with a last-minute rally on Saturday night in a sweltering auditorium that Trump said hit "110 degrees," ruining his "brand-new, beautiful suit." Former President Obama took the district by several percentage points in 2008.

“A vote for Danny boy and the Democrats is a vote to let criminals and drugs pour into our country,” Trump said. “And to let MS-13 run wild in our communities. And you know what they do once they’re there.”

The president went on to describe O'Connor as a "puppet" for House Minority Leader Nancy Pelosi, D-Calif. O'Connor once vowed not to vote for Pelosi to again lead the party, but later backtracked, saying he would support the 78-year-old if it was necessary to retake the House.

Trump also defiantly questioned the idea that historically, the party that controls the White House suffers significant losses in the midterms, declaring, "but I say why?"

Balderson narrowly won the Republican primary for the seat earlier this year amid a crowded field, and campaigned largely as a loyal Trump supporter. He said he would help "build Trump’s wall and defend Christian values" in an advertisement.

Ohio Gov. John Kasich -- who often has criticized Trump since running against him for the GOP presidential nomination -- also has endorsed Balderson, saying he is a "partner in turning around Ohio as we passed tax cuts and balanced the budget."

WATCH: TRUMP SLAMS DEMOCRATIC CANDIDATE INCRUCIAL OHIO ELECTION IN SWELTERING RALLY

“It’s really kind of shocking because this should be just a slam dunk [for Republicans] and it’s not," Kasich said on ABC News’ “This Week” on Sunday, adding that he believed Balderson would win.

“But it really doesn’t bode well for the Republican Party because this ... shouldn’t even be contested," Kasich said.

Balderson has other high-profile Republican support. Rep. Jim Jordan, R-Ohio, also briefly took the stage during the rally at the request of the president, and was met with chants from the crowd supporting him for the House speakership.

Top Republicans hoped that Balderson's support from both moderates like Kasich and avowed Trump supporters in Washington would draw a large, diverse crowd of supporters to the polls on Tuesday. Balderson himself touted his array of endorsements, telling reporters this week he had "unified the Republican Party."

The Aug. 7 special election to replace retired Rep. Pat Tiberi will be seen as a bellwether for the upcoming midterms in November, and a clear referendum on Trump and Republican leaders including Jordan.

A Quinnipiac University Poll found that Democrats have opened a 12-point lead on the generic House ballot -- a gap that Republicans had almost closed just a few months ago.

Several other states will hold primaries on Tuesday, but Ohio's special election promises to be the last major partisan test ahead of the November midterms.

The array of midterm-season nomination contests has proven confusing even for Washington insiders. Trump last week erroneously told voters in Ohio to "get out and vote" for Rep. Steve Stivers, who already had won the GOP nomination for his House seat earlier this year. He later deleted the tweet and posted his endorsement of Balderson.

Fox News' Adam Shaw, Elizabeth Zwirz and The Associated Press contributed to this report.