"Congressman Murphy has accepted a prohibited gift and used his official position to secure himself personal and political financial benefit," the letter submitted by the McMahon campaign reads.

At issue is an increase in Murphy's mortgage from Webster Bank in Connecticut, at a low interest rate. The increase came just a year after Murphy faced foreclosure on his home due to missing a number of payments on his mortgage, then owned by another bank.

McMahon's campaign stipulates that both the increase Murphy received on his loan and the rate at which it received the funds were unusual and perhaps inappropriate, considering the poor state of the housing market and Murphy's previous financial troubles.

McMahon's campaign also points out that Murphy received contributions from the bank's political action committee in 2008, and served on the Financial Services Committee in Congress, which regulates banks. Murphy had also worked for the bank during his time as a lawyer — all details which McMahon's campaign thinks indicate Murphy's loan was received as a result of an inappropriate relationship with the bank.

Webster Bank issued a statement in response to McMahon's claims insisting that there was nothing inappropriate about Murphy's loans.

In recent days Murphy's campaign has attempted, in response to the McMahon campaign's attacks on what it's calling a sweetheart deal, to focus attention on McMahon's own financial difficulties. In the 1980s, McMahon and her husband failed to pay taxes for five years and ultimately filed for bankruptcy.

Though she mentions the bankruptcy on the campaign trail as evidence of her familiarity with financial hardship, McMahon has shared few details concerning the situation. Murphy spokesperson Taylor Lavender called the ethics complaint a "desperate" attack and part of her "shameless political ploys."

"Let's remember, it's Linda McMahon who declared bankruptcy following a series of failed investments and shaky tax shelters and refused to pay taxes for five years. McMahon is still refusing to explain how much and to who she still owes," she said.

The race for Connecticut's open Senate seat remains close, and both candidates hope that issues like these — which they've both characterized as shedding light on the opposing candidate's character, or lack thereof — could help move the needle in their favor.

--This article was published at 8:59 a.m. and updated at 10:01 a.m.