A pair of third-party groups that have not disclosed their funding began airing attack ads Tuesday as the high-profile contest for Maryland’s open Senate seat entered its final two weeks.

A union-backed group called Committee for Maryland’s Progress purchased broadcast airtime in Baltimore to run an ad that argues Rep. Donna F. Edwards is “ranked one of the least effective members of Congress.” Another group called Working For Us began a new spot claiming Rep. Chris Van Hollen crafted a loophole for the National Rifle Association.

Both ads fail to offer a fully accurate picture of the candidates’ records, and both have been put on the air by organizations that have not disclosed donors — illustrating the limited clarity voters often have when assessing last-minute political advertising.