In a speech defending his signature "Medicare for All" plan, Bernie Sanders will call on his Democratic presidential primary opponents — and the Democratic Party — to pledge to reject any contributions from pharmaceutical or insurance executives, lobbyists, or PACS exceeding $200.

In a preview of the Wednesday address obtained by CBS News, the independent Vermont senator will argue, "You can't change a corrupt system by taking its money […] Candidates who are not willing to take that pledge should explain to the American people why those interests believe their campaigns are a good investment."

He's hoping the pledge will put some of his competitors on the ropes.

Get Breaking News Delivered to Your Inbox

Vice President Joe Biden, the candidate most commonly polling above Sen. Sanders, did attend a fundraiser hosted by a healthcare executive in May. Dr. Cynthia Telles, a Kaiser Foundation board member, hosted the event at her Los Angeles home. However, according last quarter's FEC filings, Dr. Telles' name did not appear on Biden's contribution list. The Kaiser Foundation, which once had an ownership stake in the healthcare giant Kaiser Permanente and Kaiser Industries, has not been associated with either since 1985.

Earlier this month, Senator Cory Booker, who has a history of accepting and rejecting pharma-linked donations, returned a $2,800 donation from a pharmaceutical executive after the contribution was uncovered by ABC News.

According to the Center for Responsive Politics, Senator Kirsten Gillibrand accepted over $150,000 from the pharmaceutical industry PACs and individuals in her 2018 senate reelection campaign. Presidential hopefuls Senators Elizabeth Warren, Amy Klobuchar, Michael Bennet, and Kamala Harris also accepted funds during their 2018 bids. Senator Sanders did as well, accepting $14,713, the lowest amounts amongst them.

Sanders' pledge exempts "rank-and-file" employees. The 2018 data from the Center for Responsive Politics does not specify if donations came from executives versus employed workers.

The pledge marks another notch in what is becoming the 2020 primary's most contested issue: What do Democratic voters want from their healthcare?

Campaigning in Iowa Monday, Biden told CBS News' Ed O'Keefe, "That's what this campaign's about, do the American people want Medicare for All or my proposal? That's what we should be debating."

On his campaign's Twitch account Tuesday, Sanders previewed Wednesday's speech, saying that he plans to explain how his single-payer system will lower prescription drug costs and make healthcare more accessible for all Americans.