Barack Obama will this weekend put his formidable “grassroots” election campaign machine into action to pressure Congress to move ahead with healthcare reform – his overriding legislative priority.

Activists in San Francisco march in support of healthcare reform, which President Obama says must happen this year

The group, Organising for America, which can draw on the database of 13m e-mail addresses built up by the Obama campaign and 2m volunteers, will hold hundreds of events that will target Republicans vulnerable to defeat in next year’s mid-term elections as well as sceptical Democratic lawmakers.