Progressive groups are warning that the Supreme Court may be on the verge of allowing federal candidates to collect multi-million dollar checks from donors.

Speaking to reporters on Monday, attorneys and representatives from the campaign finance watchdog groups Democracy, Public Citizen and Demos all raised the specter of candidates hosting $1 million-a-plate fundraisers in the near future if the Supreme Court strikes down a key provision of campaign finance law.

"The court will recreate the system of legalized corruption," said Fred Wertheimer, president of Democracy 21. "It may well open the door to striking down all of the remaining contribution limits and taking us back to the robber-baron era."

Their concerns stem from the case McCutcheon v. FEC, which the Supreme Court is set to hear next month.

In the case, an Alabama businessman is challenging the federal government's total aggregate contribution limit to candidates — arguing that it's an unconstitutional infringement on freedom of speech.

Currently, the cap on donations to all federal candidates from a single donor is set at a total of about $50,000 to campaigns and about $75,000 to parties and political action committees.

That cap is separate from the contribution limits placed on donors giving to individual candidates and political parties, which are set right now at $5,200 per donor for the primary and general election combined. Parties are currently permitted to raise $32,400 from each donor.

The progressive groups and their attorneys argue that the aggregate contribution limit acts as a hard limit on the amount of funds campaigns are allowed to raise by partnering with other campaigns.

"In particular, elimination of aggregate limits would allow candidates and officeholders to use joint fundraising committees to solicit six- and seven-figure donations from single donors," the groups wrote in an amicus brief filed this summer on the case.

Read the Amicus Brief to the Supreme Court in McCutcheon v. FEC