WASHINGTON – Groups independent of the likely nominees in Colorado’s Senate race are running ads that question the motives of the candidates, adding millions to what outside groups are already spending on negative television spots six months before Election Day.

The latest comes from the League of Conservation Voters, which announced this week it would spend almost $1 million to criticize Rep. Cory Gardner, the expected Republican nominee, as a figure who takes policy positions in exchange for support from his political patrons. In the 30-second ad, the environmental group says Gardner is beholden to billionaire industrialists Charles and David Koch and is “too extreme for Colorado.”

That ad is retaliation for an ad the Koch-backed Americans for Prosperity is running criticizing Democratic Sen. Mark Udall for supporting the national health-care law.

Both ads are misleading, but it might not matter. The Colorado race is expected to be hard-fought and nasty. Strategists from both parties see it as one of a handful that will decide the balance of power in the Senate. Democrats have a six-seat majority to defend, and a path to Republican control could run through Colorado.

Democrats have made opposition to the Kochs and their vast network of political operations a central part of their strategy heading toward November. Americans for Prosperity has its own factually challenged ads on the air.

The group claims Americans are paying more for health care but receiving fewer services. Insurance premiums have consistently increased for the last 15 years, but the Kaiser Family Foundation’s survey finds those who purchase insurance through their employer have seen a slower rate of increase than before the health-care law was in place. And there is no indication that Americans are getting fewer services under the national health-care law.