On Tuesday, voters in Pennsylvania's 18th district will elect a new congressional representative to replace Rep. Tim Murphy (R), who resigned last fall amid scandal.
This race is attracting outside attention and outside money, as it is a district that Donald Trump carried by 20 points back in 2016.
If Connor Lamb beats Rick Saccone in Pennsylvania tonight, it will not be a Democrat beating a Republican. But almost as much of the district is suburban and includes some of the swing voters that Republicans will have to convince if they are going to hold on to the U.S. House nationally.
Lamb has raised more than four times the money that Saccone has, with about $3.8 million in direct contributions compared to the Republican's $900,000.
Outside Republican groups have spent more than $8 million, vastly outspending Democratic groups on ads and get-out-the-vote efforts.
Trump Jr. admitted they can not use his father's new slogan for 2020, "Keep America Great", which he unveiled in a rally in Pittsburgh Saturday night, if his voters become complacent.
He added, the economy is set to get even better with jobs and wages going up. David Avella, a GOP strategist, said that this election has nothing to do with Trump-but that Trump should take credit if Saccone pulls it out.
Trump has put his full force behind Saccone.
Saccone, meanwhile, portrayed himself as an underdog, as national Republicans quietly blamed him for the closeness of the race.
Saccone has tried to excite voters by embracing Trump.
By Tuesday morning, Republicans were openly fretting that Lamb might win the race, and rattle the president's party ahead of this year's midterms.
Trump's footprints are all over southwest Pennsylvania. "In order to win in a tough environment we need good candidates who run strong campaigns". Kellyanne Conway, an adviser to the president, appeared at several events with the candidate on Thursday, while Donald Trump Jr. helped rally a large crowd of volunteers in Elizabeth, Pennsylvania, on Monday night.
But it's not at all clear that such campaigning persuades Trump voters to vote for someone other than Trump.
Over the weekend, Lamb celebrated an endorsement from the United Mine Workers, a union that sat out the 2016 election rather than endorse Trump or Hillary Clinton.
The AFL-CIO counts 87,000 voters from union households - around a fifth of the electorate. That number includes about 17,000 steelworkers.
"All the Republicans that I know in leadership, they're for me 100 per cent", Saccone said.
For the White House and its Republican allies, a Tuesday loss would represent both a profound embarrassment and major cause for concern in the broader push to defend majorities in the U.S. House and Senate. "The Democrats ... they're throwing everything they can at this race", he said.
Taxes: Backed the GOP tax bill. But his momentum goes beyond that, according to Trump supporters, who are coming around to the sobering reality of a Democratic wave. It will be a moderate beating a more extreme candidate. Since the beginning of March, tax ads have been essentially non-existent. "This will be a wake-up call - win or lose for Republicans - this should be a wake-up call no matter what the result is".
The excited supporters included Lamb's middle school football and basketball coach, Joe DelSardo, who recalled Lamb as "a leader from the beginning".
The most money spent on a special House race - or any House race - was $55 million previous year in suburban Atlanta, in Democrats' failed effort to take a GOP-held seat. He says he wouldn't vote for Pelosi for party leader.
Health care: He says he wants to repeal Obamacare and his campaign website calls for utilizing "free-market principles to fix our health care crisis". Lamb supports gun-owners rights and opposed a proposal to raise the minimum purchase age for a rifle to 21, but backs expanding background checks.