Washington (CNN) Democratic lawmakers pressed the National Rifle Association for more information around millions of dollars in donations the group funneled towards the campaigns of President Donald Trump and Republican congressional candidates in a letter to the NRA's president Wednesday.

The letter, from Sen. Sheldon Whitehouse and Rep. Jamie Raskin, follows a series of news reports that raised questions about the NRA's relationship with apparent shell companies that purchased political ads in potential violation of campaign finance law.

According to those reports -- which appeared in outlets including Politico, The Daily Beast and The Trace, a digital news organization funded in part by money from a gun control group -- the NRA doled out more than $50 million in support of Trump's 2016 campaign, and more than $25 million for Republican candidates in 2014, through political consulting firms that were connected to firms employed by the Republican campaigns, suggesting illegal coordination between the NRA and the campaigns, the lawmakers write.

"Any expenditure made in coordination with a candidate is considered to be an in-kind contribution to the candidate," Whitehouse and Raskin write. "The relationship between the firms the NRA employed and the firms the Trump Campaign and other Republican candidates used suggests illegal coordination."

Advocacy groups, including the gun control legal group founded by former Rep. Gabby Giffords, have filed a number of complaints with the Federal Elections Commission detailing the alleged violations.

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