On Tuesday, voters in southwestern Pennsylvania will go to the polls to chose who will replace Rep. Tim Murphy, the Republican congressman who resigned in October in the face of reports of an extramarital affair.

Tuesday’s special election, an otherwise minor race that will fill the seat in Pennsylvania’s 18th congressional district just until the midterm elections this November, has garnered outsized national attention.

Southwestern Pennsylvania is Trump country — blue-collar, 94% white, home to a number of coal mines that have in recent years shuttered — and yet the race is a toss-up — in large part because of the appeal of the Democratic contender, 33-year-old attorney and former Marine Conor Lamb.

Lamb is up against Republican Rick Saccone, an archconservative state legislator who has described himself as “Trump before Trump was Trump.” Saccone is neck-and-neck in the polls with Lamb, who is himself something of an anomaly: an Ivy League boy who served with the Marines overseas; a Democrat who is also pro-gun and willing to admit his personal (though not necessarily political) opposition to abortion.

Here’s what you should know.

Who is Conor Lamb?

Raised in the Pittsburgh area, Lamb is the youngest political figure in a family long active in southwestern Pennsylvania politics. He graduated from the University of Pennsylvania and then its law school, then joined the Marines, where he was sent to Okinawa, Japan, to prosecute military cases. After a clerkship stint in New York, he returned to Pittsburgh to serve as an assistant U.S. attorney. There, he made his name targeting the use and trade of illegal drugs, particularly opioids. State Democrats chose him as the party’s nominee in the special election in November, weeks after Murphy’s resignation.

What are Conor Lamb’s positions?

Reasonably moderate, at least by the standards of today’s Democratic Party. He’s not a vocal advocate of greater gun control and publicly seems ambivalent on the matter of abortion: a Catholic, he’s spoken out against certain pieces of pro-life legislation while also echoing his personal unease with the practice. His bread-and-butter issues are supporting labor and combating the opioid crisis. He’s already earned the support of a number of unions in the region.

Why is the Pennsylvania special election getting so much attention?

Because it’s widely seen as a barometer of Trump country politics ahead of the crucial 2018 midterm elections. Trump won Pennsylvania’s 18th congressional district by 19 points in 2016, pushing the swing state in the direction of a crucial electoral college map victory for the now-president. The race also comes at a pivotal moment for Trump’s brand of populist protectionism, as he clamors for a trade war built around strict tariffs on steel and aluminum. (The move is widely unpopular among members of his own party.) The working-class district is home to roughly 17,000 voters who are either steelworkers or their relatives, the sorts of folks Trump pledged to prioritize during his presidential campaign.

Why isn’t Rick Saccone winning easily?

Given the pro-Trump numbers in the area, it would seem like Saccone should be a shoo-in. But a number of recent polls have Lamb an inch ahead of Saccone — three points, according to a survey cited in local media. Lamb has garnered widespread appeal in the region as an old-school Democrat: a vocal supporter of blue-collar labor and the unions that support it; ostensibly less concerned with the social politics that have occupied the mantle of the party’s agenda in recent years. (He recently said he would not support a ban on assault weapons, for instance.)

Which is precisely why this race is so interesting. Should he win, Lamb’s success could be cited in support of two theories: one, that the Democratic Party hasn’t entirely alienated its traditional working-class base, as many have ventured after 2016, and two, that the appeal of Trump’s cult of personality is limited as a political force. (This second point was raised in December, after the seismic loss of the evangelical Trumpist Roy Moore to Democrat Doug Jones in Alabama’s special Senate election.) Trump spent Saturday in Pennsylvania campaigning for Saccone, but it’s unclear what, if anything, that will do to bolster the Republican’s chances.

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