"Much has been made of whether striking Montana's contribution limits is good policy and good for Montana voters. This case, though, is not about policy. It is about following the law that the United States Supreme Court set out," Lovell wrote.
The Montana attorney general's office did not immediately comment.
One of the attorneys working for the Washington D.C.-based American Tradition Partnership, a group at the center of several Montana cases, said they hope the appeals court will lift its stay in the wake of Lovell's full decision.
The attorney general's office argues that American Tradition Partnership is a shadowy front group which is illegally trying to conceal its political spending, perhaps with money received from foreign corporations. The state is seeking sanctions against the conservative group in a separate court case.
Montana has seen many of its laws struck down in the wake of the Citizens United decision that opened the door for more corporate spending in federal races, citing freedom of speech issues.
Last month, a federal appeals court struck down Montana's ban on partisan endorsements of judicial candidates, citing Citizens United.
U.S. District Judge Charles Lovell earlier this year ruled as unconstitutional laws requiring attack ads to disclose voting records and a ban on knowingly false statements in such ads.
The Supreme Court also tossed the state's century-old, voter-approved ban on independent corporate political spending in state races.
That decision prompted a new ballot initiative that, if approved in November by voters, asks state leaders to seek a constitutional amendment undermining the high court's decision.