WASHINGTON -- We Are Kentucky, a super PAC formed to support the Senate candidacy of Kentucky Secretary of State Alison Lundergan Grimes (D), raised $260,500 in its first six months of operation, according to a report filed Wednesday with the Federal Election Commission.

Labor unions provided the majority of the funds for the pro-Grimes super PAC. The United Auto Workers and the United Association, a union of plumbers and pipefitters, each donated $100,000 to the group. Jerome Kohlberg, a co-founder of the private equity powerhouse Kohlberg Kravis Roberts & Co., gave $50,000; a trust in his wife Nancy's name gave another $50,000.

The next largest donation came from Salt Lake City venture capitalist Ryan Smith, an increasingly big donor to Democratic groups and causes. Smith gave $10,000 to We Are Kentucky.

Grimes is challenging Senate Minority Leader Mitch McConnell (R-Ky.) as he seeks a seventh term in the Senate.

There are also independent groups backing McConnell and making large advertising buys. A former McConnell political director, Scott Jennings, launched the super PAC Kentuckians for Strong Leadership in 2013 and took the helm of a nonprofit called the Kentucky Opportunity Coalition. Those two groups have already combined to spend more than $2 million to promote McConnell and attack Grimes.

Kentuckians for Strong Leadership reported raising nearly $1.2 million in the first half of 2013 from big-name Republican donors like the now-deceased Texas homebuilder Bob Perry and New York marketing executive Philip Geier. The group must file its FEC disclosure report covering the second half of 2013 by Jan. 31.

The Kentucky Opportunity Coalition, as a social welfare nonprofit, is not required to disclose its donors. It is one of a number of new "dark money" nonprofits popping up in states with contested Senate elections in 2014.