MT. LEBANON, Pennsylvania — Army combat veteran Sean Parnell kicked off his race as a Republican running for the U.S. House here at Pamela’s Diner. It's within a stone’s throw of the district office of Rep. Conor Lamb, the Democrat he is challenging for the 17th Congressional District seat in 2020.

“My plan for today is just to tell the voters who I am, give them a sense of what I stand for, and just listen to them and figure out what they feel is important,” said Parnell before he headed off to two other diners in the district in Beaver and Butler counties in the western and northern suburbs of Pittsburgh.

The district is made up of neatly trimmed Allegheny County upper-middle-class, left-leaning suburbs such as Mt. Lebanon, working-class communities in Beaver County packed with labor families, and fiscal conservatives and enough rural and exurban Butler County voters to make the whole district a smidge Republican leaning.

Parnell spent his time talking mostly with customers but took a beeline to the kitchen before he left to talk to the cooks and servers. “I worked at Smartie Arties as a bus boy, then as a cook from the time I was 15 until I was 21. I know how hard that job is and I just want to thank them,” he said.

Parnell was born in the Oakland neighborhood of Pittsburgh and went to Greensburg Central Catholic High School in Westmoreland County. While attending Clarion University, he watched events unfold on 9/11 and said he felt a call to duty to join the Army. His service began in Afghanistan in 2006, commanding a platoon called "The Outlaws," who were stationed near the Pakistani border.

The experience and his bravery there earned him two Bronze Stars, one for valor, and the Purple Heart, as well as a New York Times bestseller, Outlaw Platoon, which captures in vivid detail his platoon's grueling 16 months spent engaging in endless firefights in the mountains of Paktika Province to upend the Haqqani network.

Lamb, the scion of a western Pennsylvania Democratic dynasty, is a former federal prosecutor and Marine Corps officer. He narrowly won his first race, beating Republican Rick Saccone for a seat in the 18th Congressional District in a special election in the spring of 2018.

Lamb then chose to run in the 17th Congressional District after the controversial move by the majority-Democrat state Supreme Court, which redrew all of the congressional lines in the middle of last year after it determined the seats were politically drawn to favor Republicans, only to turn around and redraw them to favor Democrats.

The next race pitted two incumbents against each other, Lamb and Republican Keith Rothfus, who lost the seat to Lamb by a whopping 12 percentage points last November.

Before Parnell’s surprise jump in, Lamb was considered to have an easy glide to the finish line next year. No one expected House Republicans to be able to recruit a quality candidate to run in any of the swing districts that the GOP lost last year that placed them in the wilderness, let alone one in western Pennsylvania.

Kyle Kondik, managing editor of Sabato’s Crystal Ball at the University of Virginia’s Center for Politics, said the Army Ranger’s entrance into the race is a good development for the GOP, “I think this makes the district more competitive than it was prior to Parnell entering the race.”

Kondik moved the race from ‘Safe Democrat’ to ‘Likely Democrat’ the night before Parnell’s announcement.

In 2018, Lamb was the center of the Democratic universe, and he raised $8.99 million in the Democratic Party’s effort to shine national attention on GOP vulnerabilities and hand Trump a House loss. Since January of this year, Lamb has raised $740,000 and has $563,000 cash on hand.

As a candidate, Lamb ran on a moderating message and avoided any support for impeachment. He benefited largely from union family support, something his Republican opponents were unable to capitalize on in the same way that President Trump did when he won the district by 2.5 percentage points.

On Thursday, Lamb cast a yes vote for an impeachment inquiry of the president.

Since the beginning of the 116th Congress, Lamb has voted over 90% of the time against legislation backed by Trump, which is probably why Trump publicly urged Parnell to challenge Lamb when he was speaking at the Shale Insight conference in Pittsburgh last week.

“It looks like PA 17 is going to have a true race in 2020,” said Jeff Brauer, political science professor at Keystone College.

“This race very well could become a bellwether race that will draw national interest and attention,” he said.

In short, if Parnell is ahead or closing in on Lamb for the western Pennsylvania seat, then Trump will likely win the Keystone State at the top of the ticket. Democratic presidential candidates cannot win Pennsylvania without western Pennsylvania support, and they cannot win the presidency without Pennsylvania.

Brauer notes that in Lamb’s first two victories, he positioned himself as a moderate in an area in which Trump garnered and maintains significant support. “Lamb avoided running against Trump as much as possible, and he even claimed that he liked Trump. However, since taking office, Lamb's actions and voting record have been to the contrary, as he has been a highly reliable Democratic vote for the House majority,” Brauer said.

This past week, Lamb voted with the Democrats on a Republican effort to prevent future presidents from banning fracking, a game-changing economic driver in western Pennsylvania.

“This time Lamb will be running against a peer," said Brauer. “In his first two elections, he ran against older, less charismatic opponents with whom he was able to easily draw a contrast. Parnell is also a young, charismatic millennial, as well as a veteran."

Brauer said Parnell can also run as a Washington outsider, as he has not run for office before. “Therefore, he can contrast himself to the Lamb family political dynasty. In many ways, Lamb will meet his match with Parnell,” he said.

Brauer said 2020 turnout will also be a different animal than a midterm turnout. “This will be Lamb's first election during a presidential year. Special election turnouts are generally low, and midterm election turnouts are usually in the 40% range, though 49% in 2018 was on the higher side than usual, while presidential elections draw in the mid-50s to lower 60s.”

A higher turnout with an incumbent president of the opposite party who had previously won the state and the district at the top of the ticket will likely hurt Lamb's chances and bolster Parnell's, said Brauer. “With all that said, Lamb still will have the advantages of incumbency, as well as a district that was made more Democratic with the court-drawn map,” he said.

Salena Zito is the author, with Brad Todd, of The Great Revolt: Inside the Populist Coalition Reshaping American Politics . Todd is a campaign strategist for Parnell.