I posted this (more or less) in a comment to an earlier post, but I'll repeat it here:
This list on ActBlue seems to correlate well with the "toss-up" races listed on the NY election map and the recent polling on TPM, at least for the Senate races and the half-dozen or so House races I checked.
Another option, which I should've thought of earlier, would be to just donate to the DNC and let them figure out what to do with the money. On the one hand, their job is presumably to crunch the numbers and figure out where it should go. On the other hand, they've been, you know, kind of sucking for the past few elections. Plus, it doesn't seem as direct, somehow.
What do you think? Just write a big check to the DNC, or smaller checks to individual races that look close?