MOUNT LEBANON, Pa. — Polls have opened in Tuesday’s special election in Pennsylvania’s 18th congressional district, where Republican Rick Saccone faces a stronger-than-expected challenge from Democrat Conor Lamb and keep a U.S. House seat in GOP hands.

An 11th hour visit by President Trump, more than $10 million of Republican campaign spending, and a coordinated labor union effort for Lamb have put the national spotlight on a district that Trump won by nearly 20 points in 2016.

The outcome carries major implications for the president and congressional Republicans fighting to hold onto their majority in November’s midterm elections. The party controlling the White House typically loses seats in the midterms and an unpopular president makes the Republican challenge even harder.

Conor Lamb, Democratic candidate for the U.S. House of Representatives, speaks during a campaign rally with members of the United Mine Workers of America at the Greene County Fairgrounds in Waynesburg, Pennsylvania, on Sunday. MUST CREDIT: Bloomberg/Andrew Harrer

Saccone spent the final hours of the campaign with Donald Trump, Jr, the president’s oldest son, rallying at a VFW hall in his hometown of Elizabeth and warning Republicans that “the left” was energized for all the wrong reasons.

“I’ve talked to so many of these [people] on the left, and they have a hatred for our president,” said Saccone. “I’ll tell you, many of them have a hatred for our country. I’ll tell you some more – my wife and I saw it again today. They have a hatred for God.”

Rick Saccone, Republican candidate for the U.S. House of Representatives, stands in front of a campaign sign during an event with volunteers and service members at the Blaine Hill Volunteer Fire Department in Elizabeth, Pennsylvania, on Monday. Bloomberg/Andrew Harrer

Lamb held no public events on Monday, focusing on door-to-door campaigning as part of a do-no-harm approach that irked Republicans.

On Tuesday, after casting his own ballot in the suburb of Pittsburgh where he lives, Lamb will hit the polls with his grandmother, then with a conservative Democrat whom he defeated for the party’s nomination.

“We’ve worked hard. We’ve had a lot of fun,” Lamb said Monday night in a tweet. “Let’s win this.”

The southwest Pennsylvania district will disappear this year, thanks to a court decision that struck down a Republican-drawn map. But the scandal-driven resignation of former Rep. Tim Murphy – a Republican who had not even drawn a Democratic challenger in his last two races – gave Democrats an opening, and Lamb raced through it.

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.

The final public poll, from Monmouth University, found Lamb ahead by two to six points depending on the depth of voter turnout. Internal Republican polling also found Saccone trailing narrowly, though picking up a little ground since the president’s Saturday visit to the district.

“My father is not on the ticket, but everything he stands for and represents is on the ticket,” said Trump Jr. in Elizabeth. “Get out there and vote.”

The Democrat, a Marine veteran and former federal prosecutor, ran as a protector of Social Security and Medicare who wanted “new leadership” to replace House Democratic Leader Nancy Pelosi, D-Calif. In the closing days, national Republicans argued that he had blurred the lines between the parties, gaining on Saccone only because he did not sound like a Democrat.

“He’s pro-gun, he’s pro-tariff, he’s pro-Trump, essentially,” Republican National Committee chair Ronna McDaniel said in a Monday radio interview. “It’s going to be a tight, tight race when you have two people running basically for the same party.”

Republicans in Pennsylvania have run with a slightly different spin, emphasizing that registered Democrats slightly outnumber registered Republicans in the rural areas south and west of Pittsburgh.

“You have to remember, this is a Democrat district, notwithstanding the fact that the president won this by 20 points,” Pennsylvania Republican Party chairman Val DiGiorgio said Monday on Fox News.

But the area covered by the 18th district had been trending Republican for decades. Starting in 2004, Republican candidates for president carried the district by larger and larger margins, peeling traditional Democrats away from their party on issues like abortion and gun rights.

For months, Republican groups led by the National Republican Congressional Committee tried to define Lamb as a national Democrat, the kind who usually lost. The bulk of Republican Party-backed TV ads linked Lamb to Pelosi, notwithstanding his criticism of her; other ads warned that he would have opposed the Tax Cuts and Jobs Act.

In the final weeks, Republicans pivoted away from that approach, attacking Lamb’s record as a federal prosecutor and targeting Democratic voters to say that Lamb did not share their values on gun rights or the minimum wage.

Lamb, whose strong fundraising let him to run only a few hundred fewer ads than Republicans, countered those attacks while counting on a massive coordinated campaign by labor unions – who have crossed with a Saccone on issues like right-to-work and pensions – to drive out votes.

Saccone, meanwhile, portrayed himself as an underdog, as national Republicans quietly blamed him for the closeness of the race. The Republican had won his party’s nomination in an upset, after winning four terms in the state house through low-dollar, grass-roots campaigns. Lamb, a first-time candidate, raised five times as much money, then attacked Saccone for the “dark money” evening things up.

Republicans in the Trump era had found similar fault with other struggling candidates. They had worried that Rep. Karen Handel, R-Ga., was on track to lose a special 2017 election in suburban Atlanta; her four-point victory went on to boost morale so much that she would hold pep talks for Republican candidates. They largely abandoned Roy Moore, the scandal-plagued nominee in last year’s Alabama Senate race who lose to Democrat Doug Jones.

But Handel, who raised funds with Trump, did not bring him to the district for a rally. Trump did not campaign in Alabama for Moore, appearing at a rally just outside the state and urging voters across the border to support him.

The White House did much more for Saccone, sending in Vice President Mike Pence, Secretary of the Interior Ryan Zinke, and White House advisers Kellyanne Conway and Ivanka Trump. An NRCC radio ad running through the election reminds voters that Trump called Saccone “a great guy.” Saccone has returned the favor by wholeheartedly praising the president, from his record on the economy to his smash-mouth approach to politics.

“He’s such a great speaker,” Saccone said Sunday on Fox News. “He’s such a powerful person that commands the audience. Everybody that’s in there is just in awe.”