RICHMOND, Va. (AP) — Virginia voters will make their picks Tuesday in primary contests that could help decide the balance of power in the General Assembly.

Both parties have some highly contested races. Virginia is one of only four states with legislative elections this year, and the only one where there's a chance of Democrats flipping control of the legislature. Republicans currently have narrow majorities in both the House and Senate.

Polls will be open 6 a.m. to 7 p.m. Each voter will need to bring a photo ID, and declare which party's ballot they want. Any registered voter can vote in either party's primaries, but can't vote in more than one.

Anyone not already registered won't be able to vote. Virginia doesn't allow same-day registration.

Here's a guide to Election Day:

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THE STAKES

The primaries have produced highly charged battles in both parties.

An unusually high number of Democratic incumbents are being challenged by newcomers in the mold of U.S. Rep. Alexandria Ocasio-Cortez, who has emerged as a leading voice for progressive policies after her upset win in a Democratic primary in New York last year.

On the GOP side, campaigns have been fueled by lingering resentment over last year's vote to expand Medicaid.

Challengers in both parties have accused incumbents of selling out while serving in Richmond. Incumbents have pointed to their mainstream appeal, saying they're best positioned to hold onto their seats for their party in November.

Virginia's 2017 elections provided an early warning of the anti-Trump blue wave that swept through the 2018 U.S. midterms. Now this year's legislative elections could signal national trends in 2020.

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TOP RACES

One of the most closely watched Democratic races involves Senate Minority Leader Dick Saslaw, a canny veteran of Capitol politics who is ardently pro-business and chummy with Republicans. Saslaw hasn't faced a primary challenger in 40 years. This year he has two opponents.

One of them, human rights lawyer Yasmine Taeb, has been painted Saslaw as too cozy with special interests and too lax about ethics in Richmond.

Nicole Merlene is making similar arguments against another Northern Virginia incumbent, Sen. Barbara Favola.

Republican Sen. Emmett Hanger, who leads the Senate Finance Committee, is facing challenger Tina Freitas. She has hammered Hanger for his vote to expand Medicaid. The state's hospitals have spent heavily to help Hanger keep his seat.

Another pro-Medicaid expansion delegate, Republican Del. Bob Thomas, also faces a primary challenger.

And in a Richmond-area Senate seat, former Del. Joe Morrissey is trying to make a political comeback against incumbent Democratic Sen. Roslyn Dance. Morrissey is a former Virginia lawmaker who used to spend his days at the state Capitol and his nights in jail after being accused of having sex with his teenage secretary.

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LOCAL RACES

In northern Virginia, two prosecutors' races have taken center stage, thanks in part to nearly $1 million from a political action committee financed by liberal billionaire George Soros on behalf of two challengers.

Parisa Dehghani-Tafti in Arlington County and Steve Descano in Fairfax County are challenging Democratic incumbents Theo Stamos and Ray Morrogh, respectively. The challengers say they want to make the criminal justice system fairer to the accused. The incumbents say they've implemented many reforms on their own.

Also in Fairfax County, multiple candidates are running for the Democratic nomination to lead the Board of Supervisors in the state's most populous jurisdiction. The incumbent, Sharon Bulova, opted not to seek another term.

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