Changing the current electoral system could pave the way for keeping "dead governments on life support machines", David Cameron has said.
Millions of people are still to be convinced about swapping the existing first-past-the-post system for the alternative vote, the Prime Minister conceded.
But he said a Yes vote for AV would usher in a system that would be "undemocratic, hopelessly unclear and much more complicated than what we have".
Speaking to an audience in Maidstone, Kent, ahead of the AV referendum on May 5, Mr Cameron said there were "obvious arguments" for sticking with the current system.
He said: "It is a system that is so clear that it can be summed up in one sentence - we all vote for our favourite and the one with the most votes wins. Simple.
"It is a system so democratic that each person's vote has the same value, has the same worth. You only get to vote once."
Lord Mandelson had earlier urged Labour supporters to "think strategically" when casting their votes in the referendum.
"If David Cameron, having opened the way to this referendum through his negotiations with the Liberal Democrats, were to see a Yes vote his party would never forgive him," he told Sky News' Boulton & Co.
"He has many critics, many detractors in the Conservative Party, they are desperately searching for some weapon to use against him and to pitch him out.
"If there was a Yes vote that weapon would be placed into the hands of his critics and I think they would use it."