VA may be a soft-blue state, but that doesn't mean this race can't swing either way. The current RCP polling average has Northam at only a +1.2 point advantage. Though non Republican-leaning pollsters put him at a +4 or +5 point lead.
In order for any candidate to win in a close election, they need their base to turn out in high numbers, (obviously) so it's important to understand how the different bases operate.
In general, older voters tend to have high turnout, probably because they view voting as a civic duty. While Millennials tend to have lower turnout, only having very high turnout for candidates that they strongly support or agree with such as Barack Obama in 2008 or Bernie Sanders recently. It is hugely important for younger voters to turn out in high numbers.
Thus, if Ralph Northam wants to win, it's of immense importance that he appeals to policies millennials (and the Democratic party at large) find favorable. Such as free college which 80% of Democrats support and roughly 2/3 of millennials (Morning Consult) and Medicare for All which 65% of Democrats support and again roughly 2/3 of millennials (Qunnipac University).
While Ralph Northam has shown support for the decriminalization of marijuana and a $15 minimum wage which are also popular among the Democratic base, his campaign focus has not been on these populist issues that would greatly increase the chances that millennials and the larger Democratic base would turn out for him in high numbers.
Instead, his ads have focused on attacking Trump or Ed Gillespie, and promoting vocational training and apprenticeships, proposals that while not unpopular are far more meek than policies like Medicare for All which enjoy widespread support.
(P.S: If you want to watch more ads, check out the Youtube channels for Gillespie and Northam, 30 seconds long and worth the time)
Furthermore, Northam's attacks on Gillespie by linking him to Trump probably won't do him much good either, as that alone didn't inspire the Democratic base to turn out in high numbers for Hillary Clinton in 2016 when it Trump literally was on the ticket. While some of Northam's attacks on Gillespie which call him a lobbyist are probably an effective way to attack him but don't translate into Northam's own base being excited to vote for him. Again, how did that work in 2016?
On the other hand, Gillespie can count on his base turning out in decent numbers because older Republican voters tend to vote more frequently, providing fairly good turnout anyway. So ultimately, it's Gillespie who will win, albeit with a probably slim margin of victory of 1-2 points.
(P.S: Do watch The Walking Dead, great show).
