* Bernanke calls stress tests fair and comprehensive

* Critics question severity of tests, capital needs

* Bank shares fall more than 4 percent (Adds paragraphs 3-4, updates share prices)

By Christian Plumb and Elinor Comlay

NEW YORK, May 7 (Reuters) - The U.S. government on Thursday prepared to draw a line separating weak banks from strong ones in an attempt to put the worst of the financial crisis behind it.

The results of "stress tests" of the 19 largest U.S. banks -- due at 5 p.m. EDT (2100 GMT) -- are the culmination of a months-long exercise aimed at reviving the financial system and are expected to show about half the banks need more capital.

The government is focused on banks filling shortfalls in their equity capital -- an amount that Goldman Sachs analysts estimated could total $130 billion this year. That estimate included what the stronger institutions would need to raise to pay back government money and funds the weaker ones need to fill shortfalls. [ID:nWNAB5331]

Share prices in banks broadly have climbed about 50 percent since the stress tests were first announced as part of a wider plan to revive the sector in February. But on Thursday, the KBW Banks Index slumped 4.8 percent, and some investors viewed the stress test results as spelling an end to that rally.

"From my perch, investors should sober up and reduce their holdings in financials now," hedge fund manager Doug Kass said in a note to clients. "Financial stocks are now priced to perfection." In an interview, Kass said he is selectively selling short some bank stocks.