(CNN) - Arizonans vote Tuesday to fill the seat left vacant by former Rep. Trent Franks, who resigned in December amid sexual harassment accusations from members of his staff.
Voters in the area -- home to many retirees in Maricopa County's Sun City -- have consistently sent Republicans to Congress. Should Democrats win here tonight, it will be a huge loss for the GOP. But given the state of the race and early ballots already in, Republicans aren't as concerned like in last month's Pennsylvania special election.
Here's what you need to know:
Polls are open 9 a.m. to 10 p.m. EDT (6 a.m. to 7 p.m. MST). Results likely won't come in before 11 p.m. EDT, as Arizona law requires votes be held for an hour unless all precincts have reported.
The candidates -- Republican Debbie Lesko and Democrat Hiral Tipirneni -- have been fighting for the seat, though spending is down significantly from Pennsylvania's special election last month.
President Donald Trump won the district by 21 points in 2016. Republicans expect Lesko to win the seat, possibly with a double-digit margin, though she most likely won't be able to match the President.
Democrats believe Tipirneni's campaigning and their extensive get-out-the-vote effort could beat expectations and lead to a surprise.
There is an automatic recount if the margin is 200 votes or less or 0.1% or less.
Here's a closer look at what to watch tonight:
PA-18 redux? On paper, a special election for a congressional seat in a district the President won by 21 points with a newcomer Democrat against a Republican state senator seems like a Pennsylvania Groundhog Day, but it's not. The spending and enthusiasm don't compare to the Lamb vs. Saccone match in suburban Pittsburgh. This district is home to a significant senior population that has largely voted already, due to Arizona's early voting laws that allow citizens to vote by mail.
Lesko is well-liked. As a state senator, Lesko worked on a lot of local issues that resonated with her constituents. From her reform work for police and fire pensions, to changing laws about the use of golf carts (a big issue there), Republicans point to her grassroots work at the local level. Advisers say she enjoys campaigning and knows how to fundraise.
The doctor is in. Tipirneni is a doctor who has spent a lot of time focusing on health care, an important issue in the district. Campaign advisers say her data-driven approach shows how she can be non-partisan, which resonates well with voters. Her strong volunteer effort included door knocking calls, and texts to bring in new voters. This could help close the 21-point gap, though few Democrats are expecting a win here tonight.
Lessons for the fall. Last month's loss in Pennsylvania was a wake-up call for Republicans. National Republican Congressional Committee Chairman Rep. Steve Stivers has been meeting with candidates who are behind in fundraising, and the committee is working with candidates to define themselves earlier than previous cycles. National Democrats are trying to stay behind the scenes and let local candidates be their own messenger. They are helping with organization and behind-the-scenes work but say local money is a bigger help than outside funds.
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