Brown has said that her work for such corporations undermines the premise of her candidacy: that she will fight for middle-class families against big business in Washington.
"Now, voters are learning that contrary to your claim to always stand with middle-class workers, you have instead chosen to stand with large corporations and against workers," Brown writes in the letter.
It's an effort from the Brown campaign to counter that main line of messaging from Warren, one that can be seen vividly in a new ad from her campaign that features local Massachusetts voters touting Warren's support for "working America."
"This lady will fight for us," one man says.
The letter also continues a more aggressive line of attack from Brown, launched last week, that questions Warren's integrity as a candidate.
"Taken together with your refusal to satisfy media demands for a full six years of tax returns and your refusal to release your university personnel files, these new revelations leave voters with a disturbing and confusing picture of whose interests you really represent," he writes.
The letter comes as a spate of new polls out over the past few weeks gave Warren a lead over Brown, indicating Warren could have the momentum in the race. But one poll put Brown ahead, and it's too soon to call the race, as Brown remains well-liked in the state and has the advantage of incumbency.