Vanguard Group Inc.’s biggest salvo yet in its campaign to cut fees for the investing masses presents industry rivals with a painful choice. Follow suit and lose potentially hundreds of millions in revenue — or hold the line and risk losing badly needed market share.

Vanguard's fee cuts may not significantly affect BlackRock, as the latter has a more diversified ETF strategy and mainly caters to institutional investors.

Vanguard Group Inc.’s biggest salvo yet in its campaign to cut fees for the investing masses presents industry rivals with a painful choice. Follow suit and lose potentially hundreds of millions in revenue — or hold the line and risk losing badly needed market share.

Vanguard made headlines by touting what it called “our largest-ever reduction in expense ratios.” The asset manager known for rock-bottom costs, said it was reducin