Casting doubt on the leadership so far shown by Mr Hague, Mr Portillo warns that Gordon Brown, the Chancellor, could whittle away opposition to Britain joining the single currency unless a lead is taken against the euro.
"Until we provide that leadership, there is a real danger that the majority against joining European monetary union will be whittled away by Gordon Brown's inglorious, but corrosive, argument that British membership is inevitable," Mr Portillo says in an article for The Daily Telegraph.
And in a sideswipe at Mr Hague, the former cabinet minister says that the Conservative Party can become a credible force again only by "showing it has something to say".
Mr Portillo writes: "Economic union is not the only question on which it must find its voice, but it is the most important issue for Britain since the last war and the electorate deserves a proper debate."
Mr Hague's failure to make a bigger impact has fanned speculation that Mr Portillo, who is rebuilding his support since losing his seat at the last general election, is waiting in the wings to take over from Mr Hague, if he can get a Commons seat in a by-election.
The anxiety within the Tory leadership about Mr Hague's lack of impact also surfaced at the weekend in reports that the Conservative Party is poised to carry out a shake-up of the party's press operation, sidelining Mr Hague's press officer, Gregor Mackay, by hiring Jonathan Holborrow, former editor of the Mail on Sunday for a six-figure sum.
Pro-European Tory MPs including Michael Heseltine believe Mr Portillo would seek to take the Conservatives further to the right, by steering the party towards a policy in favour of eventual withdrawal from the European Union.