They are collating a list of volunteer shareholders to put their names to an action under the Companies Act.
That allows holders of 5% of shares or 50 shareholders to go to court to overturn a resolution to take a company private, according to SU.
A spokesman said: "We want to take action to prevent Glazer implementing his plans to take the company private and load it up with excessive amounts of debt, with no realistic prospect of playing it back.
"SU and the small shareholders will be aiming to block his ambition every step of the way, having taken the best legal advice available."
The pressure group claim that the full extent of Glazer's borrowing is "more horrifying than they first imagined."
The SU spokesman said: "Glazer's business plan projects EBITDA (operating profits) in the year to 31.07.2006 of £57.1 million and in the next year to 31.07.2007 of £89.1 million.
"Where can he find another £32 million of operating profit in one year's trading?
"Our analysis of Glazer's financings before refinancing the PIKs (preferred securities) shows negative net cash in year two and little net cash in year three.
"That situation carries major implications for Sir Alex Ferguson (or the next manager) in terms of player transfer budgets and contracts."
SU are also setting up the Phoenix Fund into which supporters can contribute some or all of the proceeds of the sale of their shares.
The fund is viewed as the launchpad for buying the club back when it comes on the market again.
"If only half of the 72,500 (Ed. there are currently around 31,000) SU members and the remaining individual shareholders put in £500 each to the Phoenix Fund, it would kick off with £36.25 million in the buyback pot, " added the SU spokesman.
"This could be the best chance that fans of a major football club in the UK will ever have to genuinely reconnect their club with the community and the fanbase."