Amazon and Alphabet are spearheading what is shaping up to be the most intense political campaign by corporate America in recent history as part of a last-ditch attempt to stop Congress from passing laws to curb their market power.

The companies are targeting a “self-preferencing” bill which would prevent large online platforms from using their dominance in one field to give other products an unfair advantage — for example, Alphabet using its Google search engine to promote its travel or shopping products.

If the bill goes through, it is likely to lend momentum to a wave of legislation aimed at strengthening America’s competition rules, in what could be the biggest update of the country’s antitrust rules in a generation.