There's a real danger on both sides, because if elections were held tomorrow the Republicans would likely lose the house because they are not winning these special elections by much. And becoming the party of "No" works, ask how the Republicans got all their gains. The economy is doing ok, but it's not seeing the massive gains (sans the Stock Market) that will help rust-belt groups that helped give us a President Trump.
My major fear/concern is that this logic breaks down if the two sides are motivated by different forces. It is absolutely true that the Republicans benefited from being the party of "No" and being as obstructionist as they could, 2012 Presidential Election notwithstanding.
I am not, however, entirely sure such tactics will work in motivating Democratic voters. Nor am I entirely convinced that Republican voters would be demotivated/switch as you outline. Even if they aren't actually helped, if they perceive they have it may not matter. (IIRC you can see how the numbers on the economy shift on partisan lines even when its state hasn't really changed all that much)
It's really unfortunate, but I think there really might be something to the whole "Democrats need to find a charismatic candidate above all else because they just refuse to turn out sufficiently otherwise, policy be damned."
but then i have been spiraling into pessimism and cynicism over these things for a while now so maybe i'm just wrong. hope i am.
Democratic voters don't want them to be the party of no. Their voters simply have different standards for their leaders. Not better or worse, just different. ---