Disney’s largest shareholder, index-fund manager Vanguard, plans to support management over Nelson Peltz’s Trian Partners in Wednesday’s board vote, Bloomberg News reported Tuesday, citing unnamed people familiar with the matter.

Institutional shareholders have until Wednesday to change their vote. Vanguard owns 7.8% of Disney shares. Blackrock, Disney’s second largest shareholder with 4.2% of shares, is also supporting the incumbent board and CEO Bob Iger, the Wall Street Journal reported Monday.

It would be a significant blow to Peltz’s ambitions to join Disney’s board if both Blackrock and Vanguard’s continue to back the media company’s candidates. That would leave only State Street and Geode Capital Management, the company’s third- and fourth-largest shareholders respectively, as unknowns.

Through an arrangement with former Marvel Chairman Ike Perlmutter, Trian controls 1.8% of Disney shares, making it the fifth largest shareholder. Retail investors have until 11:59 p.m. ET Tuesday to submit their vote by phone or online.

Trian has won support from other, smaller shareholders, including Neuberger Berman and CalPERS. For its part, Disney has called in some of the most prominent names in the corporate and media world, including JPMorgan Chase CEO Jamie Dimon and Star Wars creator George Lucas.

Disney’s shareholder meeting begins Wednesday at 1 p.m. ET.

Vanguard declined to comment to CNBC.

Read the full Bloomberg report here.