Sir Menzies Campbell has made little impact with the public and must "shape up", a Liberal Democrat Welsh Assembly member has said in a scathing attack.
Sir Menzies says he has the vision to be a good leader
Peter Black says the Lib Dem leader will have nobody to blame but himself if things go wrong.
Sir Menzies has admitted he faces a test of his authority over tax plans at the party's conference in September.
Mr Black's broadside comes after a poll suggested Lib Dem support had fallen to its lowest level since 2002.
The Guardian/ICM poll showed the Lib Dems down to 17%, with the Conservatives on 39% and Labour 35%.
ICM interviewed a random sample of 1,001 adults by telephone last weekend.
In his blog, Mr Black says his party's opinion poll ratings are stagnating and slipping back - despite Lib Dem successes at the Dunfermline and Bromley by-elections.
The Welsh AM was among those who earlier this year said Sir Menzies should face a contest to become leader, when it looked as if he was going to be elected unopposed.
In a blog entry headlined "Time for Ming to shape up", he says people had a right to expect results after Sir Menzies replaced Charles Kennedy.
Peter Black says the tax plans are unfocussed
But he has "made little impact with the public at large".
Mr Black complains that the party leadership has "established a lock on the party organisation irrespective of the party's democratic structures".
"Ming is playing for very high stakes and in doing so has placed a great deal of his personal authority on the line," he says.
"If things go wrong then there is nobody else to blame, he has made sure of that by the way he has gathered all the threads around him."
Mr Black says the next few months will be critical as Sir Menzies tries to win support at the party's conference for his tax plans.
The plans are expected to include "a long term ambition" of doubling allowances so no tax is paid until someone earns £10,000.
In the short term they will promise to scrap the 10p starting income tax rate, in effect raising the tax-free personal allowance from about £5,000 to £7,000.
They would also raise National Insurance starting thresholds to similar levels.
The basic rate of income tax will also be cut from 22p to 20p, with the threshold at which the top 40% rate of tax is paid raised from £33,000 to £50,000.
Mr Black says the proposals announced so far are "worthy" but seem "unfocussed and lack a clear narrative".
"The debate will, of course be critical but so too will Ming's performance at the conference," he says.
"We need to get some bounce in the polls out of that week in Brighton. When Simon Hughes said that Ming had until the end of the conference season to prove himself he was absolutely right."
He adds: "It is now time for Ming Campbell to start delivering on his promises and the expectations of success that are associated with him."
Lib Dem headquarters have yet to respond to Mr Black's criticisms.
Sir Menzies recently brushed off a poll which suggested that twice as many people believed Mr Kennedy would be a better leader than him.
The 65-year-old told Newsnight he "would not be judged by opinion polls after a few months".
"If I didn't think I had the energy, the values and the judgement to be the leader of the Liberal Democrats, I most certainly wouldn't have offered myself for the job," he said.