The 1983 Mental Health Act would be replaced with new laws tackling “unnecessary detention” under Tory plans for England and Wales.
The Conservatives also pledged 10,000 more NHS mental health staff by 2020 and to tackle discrimination against those with mental health problems.
But Health Secretary Jeremy Hunt told the BBC that a failure to hit A&E targets was “not acceptable”.
Opponents said without extra money, the Tory pledges were based on “thin air”.
The Conservatives say their mental health plans are motivated by the concern that “vulnerable people are being subject to detention, including in police cells, unnecessarily” as numbers of people detained or “sectioned” under the Mental Health Act have risen.