MPs have backed government plans to hold a UK-wide referendum on changing the voting system next year.
Voters would be asked if they wanted to keep "first past the post" or switch to the "alternative vote", which ranks candidates in order of preference.
But it is not certain the bill will become law before Parliament is dissolved ahead of the election.
The government says change is needed to restore trust in politics, but the Tories say it is a waste of £80m.
MPs backed the referendum plan by 365 votes to 187 - a majority of 178 for the government.
A Liberal Democrat amendment to hold a referendum earlier and on a different voting system - the single transferable vote - was defeated by 476 votes to 69.
The government put forward its plan for a referendum to be held by the end of October 2011 in an amendment to the Constitutional Reform Bill.
But the wide-ranging bill has to go through various parliamentary stages before becoming law and is expected to face opposition in the House of Lords. Downing Street has also admitted "time is tight" to change the law ahead of a general election, widely expected in May.
At one stage during the debate, former Beatle Sir Paul McCartney appeared in the public gallery for about five minutes - he chatted to a group of students from Argentina who spotted him before he left.
Opening the Commons debate, Justice Secretary Jack Straw said trust in Parliament had been "profoundly damaged" by the expenses scandal.
Part of restoring trust must be considering which voting system could best serve them, he said.
"This is an important debate. This subject is a fundamental plank of our democracy and it comes at a time when this House is held in dangerously low regard," said Mr Straw.
"The alternative vote takes on the considerable strengths of our system and I suggest, builds on it. We propose a referendum because we believe it is not for us to decide, but it is important the people should have that choice," he added.
But several Tory MPs stood up to ridicule the idea - suggesting it was an act of "political cynicism" by Gordon Brown ahead of a general election and the issue was "utterly irrelevant" to most people.
They have accused Labour of using the issue to make overtures to the Liberal Democrats in the event of a hung Parliament.
Mr Straw's Conservative shadow, Dominic Grieve, said it would cost "£80m for a gimmick which the government wishes to foist on the electorate" at a time when "every pound matters".
He suggested the prime minister had had a "belated conversion to the cause of electoral reform which he has so successfully and personally obstructed for over a decade".
The current system delivered "clear, clean results" and allowed voters to "get rid of" MPs they did not want. He said proportional representation systems "saddle a country with impossible legislatures where you cannot have any proper governance carried on at all".
For the Lib Dems, David Howarth said the government's "deathbed conversion" to electoral reform did "look like a manoeuvre".
He said his party would support the government's amendment but only so it could try to amend it "radically" in favour of a referendum on a "more proportional" system.
Mr Howarth said: "We cannot go on with a political system under which unpopular governments are elected by a little more than a third of those voting and who push through policies that two-thirds of those voting have just voted against."
Some MPs suggested other voting systems should be considered - including the French system in which the top two candidates take part in a run-off election if no-one gets 50% of the vote.
The Tories say the current system results in stable governments and keeps out extremists - if they win the general election, expected to be held in May - they are expected to scrap any plans for a referendum.
Mr Straw has also said the government will back a Tory amendment which would guarantee general election votes are counted on polling night.
Some Labour MPs also believe the "alternative vote" would benefit the least unpopular - rather than the most popular - candidates, and could cost Labour seats.
But they believe the chances of the referendum becoming law are slim. One told the BBC: "It's dead before it's even started - so what's the point?"
Labour pledged a referendum on electoral reform in its 1997 election manifesto but the idea was kicked into the long grass by Tony Blair after his landslide victory.