The Alabama Senate special election and the Virginia gubernatorial race are near-certain to dominate political pundits' attention this fall. But they could be poor predictors of 2018. The former is taking place in a deeply red, racially polarized state that bears little resemblance to most swing House districts. The latter is unfolding in a purple state, but the GOP nominee, Ed Gillespie, holds uncommon appeal with upscale voters who couldn't stomach Donald Trump.

The races that could best harbingers of November 2018 aren't likely to get much national coverage at all: all 100 seats in the Virginia House of Delegates are up for election this November.

Democrats aren't likely to pick up the chamber: they currently hold just 34 seats and would need to gain 17 to win control. Hillary Clinton did carry 17 seats held by Republicans last fall, but many of those are located in transient outer suburbs where Democratic-leaning minorities and young voters tend not to vote in off years. Moreover, Democratic Lt. Gov. Ralph Northam isn't blowing out Gillespie, so down-ballot Democrats may not be riding long coattails.

Still, if Democrats managed to pick off 10 or more GOP-held seats, it would send a signal that voters are in the mood to punish President Trump and Republicans - a mirror image of the GOP legislative gains in 2009 that foreshadowed Republicans taking back the House in 2010.

Democrats have made a lot of hay out of special election upsets this year in deep red legislative districts in New Hampshire and Oklahoma. But, those races have featured infinitesimal turnouts. For example, on Tuesday, Democrats flipped a state House district east of Manchester, New Hampshire, that gave Trump 59 percent of the vote last fall. But only 1,804 voters cast ballots in a district that cast 10,023 ballots last November.

As those elections show, Democratic voters are hyper-motivated right now. But their chances fall as turnout rises. On June 20, Republican Karen Handel defeated Democrat Jon Ossoff in Georgia's 6th CD after a $50 million contest that generated 56 percent turnout. The very same day, Democrat Archie Parnell came within three points of a shocking upset in a South Carolina race that attracted a fraction of the money and generated 19 percent turnout.

Virginia's House of Delegates races are a better proxy of what's to come in 2018. Turnout won't be too hot (as in Georgia) or too cold (as in New Hampshire). It will be driven by the governor's race, which better approximates what a midterm turnout looks like.

Moreover, there are plenty of vulnerable GOP Virginia delegates sitting in districts where Clinton outperformed Obama. Republicans like Dels. Jim LeMunyon (67th), Scott Lingamfelter (31st) and Tag Greason (32nd) all fit that bill - just as there are plenty of House Republicans sitting in districts where Trump is uniquely unpopular. Their races will test voters' inclination to send a message to Trump regardless of whether they know and like the local GOP candidates.

The five likeliest Democratic pickups are all in the highly-educated Northern Virginia suburbs: two are open seats (the 2nd and the 42nd) that are near-certain to change parties, and three more involve GOP incumbents who have only narrowly won in the past few cycles.

Of all the delegate races, the biggest potential national story lies in the 13th District, situated in the Prince William County suburbs. Democrat Danica Roem, a transgender woman and local journalist, is challenging longtime GOP Del. Bob Marshall, a steadfast social conservative who has handed out plastic fetuses on the floor of the House of Delegates and refuses to acknowledge Roem as a woman. Clinton carried the 13th District by 14 points.

Another bellwether race, and the only in Southwest Virginia, is in the 12th District between moderate GOP Del. Joseph Yost and Democratic news anchor Chris Hurst, whose girlfriend, reporter Alison Parker, was fatally shot on live television in Roanoke in 2015.

Think of it this way: if Democrats pick up five seats or less, Republicans would breathe a sigh of relief. If Democrats pick up five to ten seats, it would suggest Democrats are in contention for the House next year. If Democrats pick up 10 to 15 seats, it would be a strong sign they're on track to pick up the House majority next year. If Democrats pick up more than 15 seats, we're looking at a potential tidal wave in 2018.

Here's a quick guide to the 25 GOP-held seats to track on November 7th, along with district-level performances in the 2013 gubernatorial race, the 2014 Senate race and the 2016 presidential race. For Democrats, the difference between winning "toss up" races and "reach" or "tidal wave" districts would be the difference between a good night and a tremendous night.

Probable Democratic Pickups:

District Republican Incumbent Democratic Nominee Region 2013 Governor 2014 Senate 2016 President 2nd OPEN (Dudenhefer) (R) Jennifer Foy (D) Prince William County McAuliffe +11 Warner +6 Clinton +17 31st Del. Scott Lingamfelter (R) Elizabeth Guzman (D) Prince William County McAuliffe +3 Gillespie +2 Clinton +7 32nd Del. Tag Greason (R) David Reid (D) Loudoun County McAuliffe +7 Warner +2 Clinton +19 42nd OPEN (Albo) (R) Kathy Tran (D) Fairfax County McAuliffe +6 Warner +2 Clinton +20 67th Del. Jim LeMunyon (R) Karrie Delaney (D) Fairfax County McAuliffe +8 Warner +6 Clinton +22

Toss Ups:

District Republican Incumbent Democratic Nominee Region 2013 Governor 2014 Senate 2016 President 12th Del. Joseph Yost (R) Chris Hurst (D) Blacksburg McAuliffe +6 Warner +8 Clinton +2 13th Del. Bob Marshall (R) Danica Roem (D) Prince William County McAuliffe +1 Gillespie +4 Clinton +14 21st Del. Ron Villanueva (R) Kelly Fowler (D) Virginia Beach McAuliffe +4 Warner +3 Clinton +4 72nd OPEN (Massie) (R) Schuyler VanValkenburg (D) Henrico County Cuccinelli +5 Gillespie +6 Clinton +4 94th Del. David Yancey (R) Shelly Simonds (D) Newport News McAuliffe +3 Warner +2 Clinton +5

Democratic Reaches:

District Republican Incumbent Democratic Nominee Region 2013 Governor 2014 Senate 2016 President 10th Del. Randy Minchew (R) Wendy Gooditis (D) Loudoun County Cuccinelli +3 Gillespie +8 Clinton +4 40th Del. Tim Hugo (R) Donte Tanner (D) Fairfax County Cuccinelli +7 Gillespie +11 Clinton +8 50th Del. Jackson Miller (R) Lee Carter (D) Manassas Cuccinelli +1 Gillespie +3 Clinton +12 51st Del. Rich Anderson (R) Hala Ayala (D) Prince William County Cuccinelli +1 Gillespie +7 Clinton +6 68th Del. Manoli Loupassi (R) Dawn Adams (D) Chesterfield County Cuccinelli +2 Gillespie +4 Clinton +10 73rd Del. John O'Bannon (R) Debra Rodman (D) Henrico County Cuccinelli +5 Gillespie +5 Clinton +7 85th Del. Rocky Holcomb (R) Cheryl Turpin (D) Virginia Beach Cuccinelli +2 Gillespie +3 Trump +1 100th Del. Rob Bloxom (R) Willie Randall (D) Eastern Shore McAuliffe +2 Warner +1 Clinton +2

Tidal Wave Territory:

District Republican Incumbent Democratic Nominee Region 2013 Governor 2014 Senate 2016 President 26th Del. Tony Wilt (R) Brent Finnegan (D) Harrisonburg Cuccinelli +17 Gillespie +20 Trump +9 27th Del. Roxann Robinson (R) Larry Barnett (D) Chesterfield County Cuccinelli +8 Gillespie +8 Trump +4 28th OPEN (Howell) (R) Joshua Cole (D) Fredericksburg Cuccinelli +5 Gillespie +9 Trump +4 33rd Del. Dave LaRock (R) Tia Walbridge (D) Loudoun County Cuccinelli +15 Gillespie +20 Trump +16 62nd Del. Riley Ingram (R) Sheila Bynum-Coleman (D) Hopewell Cuccinelli +9 Gillespie +7 Trump +6 83rd Del. Chris Stolle (R) David Rose-Carmack (D) Virginia Beach Cuccinelli +4 Gillespie +9 Trump +8 84th Del. Glenn Davis (R) Veronica Coleman (D) Virginia Beach Cuccinelli +1 Gillespie +5 Trump +5

Past election results courtesy of The Virginia Public Access Project. Cook Political National Editor Amy Walter and Cook Political Report Web Editor Ally Flinn contributed to this story.