With just six weeks left to the crucial November election, a handful of battleground races for seats in the U.S. House of Representatives have captured an outsized share of campaign cash in the 2018 midterm cycle.

Much of that cash is coming from political action committees, or PACs, and other so-called "soft money" groups that can accept unlimited contributions, as long as they don't coordinate their spending with individual candidates.

The money is funding a blizzard of television and digital ad campaigns aimed at flipping control of Congress from Republican to Democratic control.