Retailer formally rejects $9.1 billion offer, amends agreement with Dollar Tree to place priority on quicker FTC approval.
The union claims the two companies underestimated the market share the combined company would have if their $8.2 billion merger is cleared by the Federal Trade Commission.
The heads of AT&T and DirecTV Groupon Tuesday twice defended their planned $67 billion merger from critics.
House and Senate hearings will be held Tuesday to examine AT&T's $67 billion plan to acquire DirectTV Group.
Move appears to be a request for an extension to wrap up CFIUS approval terms.
Expected divestitures convinced second suitor, widely believed to be Kroger, not to best Albertson's $9 billion offer.
House panel hears worries of programmers, small cable operators and Internet backbone providers.
Advocacy groups wanted a full investigation of privacy concerns raised by the companies' $19 billion merger.
The Democratic majority sitting on the Federal Communications Commission on Monday brushed aside a furious campaign by broadcasters and voted to restrict local television partnerships that critics say have been used to circumvent federal limits on local TV station ownership.
Whether food distribution is a national market will be key to the agency's view of the merger.