(CNN) Voters in southwest Pennsylvania are heading to the polls Tuesday for a special election. Democrats, looking to take back the majority in Congress next year, are hoping a high-profile win in Pennsylvania's 18th District will give them a boost in the heart of Trump country. Republicans are hoping to avoid an embarrassing defeat in a district that has voted Republican since 2002.

Here's what you need to know:

Polls in the district are open from 7 a.m. to 8 p.m. EDT.

The candidates -- Democrat Conor Lamb and Republican Rick Saccone -- are in a tough fight in which outside groups have spent over $12 million '

The seat opened up when former GOP Rep. Tim Murphy resigned in October after the anti-abortion Republican allegedly urged a woman he was having an affair with to have an abortion.

President Donald Trump won the district by about 20 points in 2016. A defeat for Republicans here won't only be embarrassing symbolically, it also will send shock waves to vulnerable Republicans around the country and could spur retirements by other Republicans in Congress.

Recounts by petitions and partial requests are allowed within five days.

Here's a closer look at what to watch tonight (and after):

THE "W" WORD: Democrats are eager to call 2018 a wave year in which they'll take back the majority in the House, and possibly the Senate. It's too early to make that call now, but if Lamb wins, it will bolster Democrats in districts that seem out of reach both due to their political makeup and fundraising. They can gain some major "Mo"(mentum) here tonight that could help bring in a big "W" in November.

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