Gubernatorial nominees Republican Ed Gillespie, left, and Democrat Ralph Northam shake hands at the end of their debate Tuesday. (Bonnie Jo Mount/The Washington Post)

The gubernatorial candidates had a debate Tuesday night. Was it a game-changer?

No. The real game-changer is Congress.

For those who did tune in, the debate between Republican nominee Ed Gillespie and Democratic nominee Ralph Northam was a rather low-key, almost collegial affair that produced neither whopping gaffes nor memorable sound bytes.

Gillespie was at his wonkish best, sticking to his major talking points on economic growth and generally doing his very best to keep President Trump at arm’s length.

Northam was calm, collected, even somewhat comforting. The incumbent lieutenant governor wasn’t long on details and did an intricate tap dance around the pipeline issue.

But both came out unscathed. Both seemed like nice guys. They even agreed on a few things. Neither moved the electoral needle in their direction.

And in a race that the polls tell us is within the margin of error, that’s a problem for both men.

Off-year elections always come down to turnout. The electorate is much smaller than in presidential election years. It is typically older and generally whiter.

That would seem to indicate a slight lean toward Gillespie.

It should have favored Ken Cuccinelli in his fight for the governorship in 2013. But the exit polling on that race showed Cuccinelli underperforming Robert F. McDonnell’s 2009 performance, even among base GOP voters.

Robert Sarvis, the Libertarian Party nominee in 2013, took some of those voters from Cuccinelli’s column, particularly among the 18-29 age bracket.

Cliff Hyra, this year’s Libertarian nominee, won’t be a factor like Sarvis was. Moreover, in Gillespie, the GOP has consciously, if narrowly, chosen a nominee who is far less polarizing than Cuccinelli was.

So, again, Gillespie should benefit.

But the most recent polls also show that Northam is doing his best where the voter crop is richest: Northern Virginia and, to an extent, Tidewater.

Fairfax County alone was able to push Democratic presidential nominee Barack Obama to a Virginia victory in 2012 and to put Democratic presidential nominee Hillary Clinton comfortably over the top in 2016.

Fairfax County delivered the 2013 gubernatorial race, too. In that race, Democrat Terry McAuliffe defeated Cuccinelli by slightly more than 58,000 votes. McAuliffe’s winning margin in Fairfax County? A bit more than 68,000.

Republicans can run up big numbers in the more sparsely populated portions of the state. But unless they can crack the Blue Wall, they can’t win.

Gillespie almost succeeded in 2014, losing to incumbent Sen. Mark Warner (D) by fewer than 18,000 votes out of more than 2.1 million statewide.

Warner carried Fairfax County by more than 53,000 votes. He needed every one to fend off Gillespie. And he also needed the help of a strong third-party challenger.

The 53,000 votes Libertarian Senate nominee Sarvis earned might not have all gone to Gillespie, and they would not have changed the math very much in Fairfax County. But if even a third of Sarvis’s statewide vote had gone to Gillespie, Gillespie would likely be in the Senate today.

Northam doesn’t have that kind of help this year.

Which means Northam, even with a Northern Virginia advantage, needs something more to avoid a Warner-like brush with defeat. And a Mason-Dixon poll spells that need out very clearly:

If African-American voters don’t show up in large numbers, something that is not uncommon in off year elections, that would significantly help Gillespie’s chances.

True enough.  But what gets those voters to the polls?

That may depend on what happens across the Potomac, where Congress is weighing another go at repealing the Affordable Care Act.

If the ACA is repealed, Gillespie bears the burden of running into the teeth of concerted, organized and energized Democratic opposition. That could get Northam the turnout he needs — in a very big way.

Gillespie knows it, which is why he bobbed and weaved around the issue after the debate.

It’s up to his congressional Republican brethren whether he can do so much longer.