What if they had an election and everyone voted?

It wouldn't matter, says political scientist Stuart Rothenberg. The outcome of recent presidential elections would not have been different even if the half of people who boycott elections had cast ballots.It used to be conventional political wisdom that Democrats suffer when people don't vote.

The thinking was that non-voters tended to come from those parts of society - minorities, blue-collar, low-income, less-educated - that traditionally supported Democratic candidates, especially in presidential elections.

But research establishes that non-voters are pretty much like people who vote, only flightier, Rothenberg said in a study published Wednesday, "What If Non-voters Voted?"

"There is no compelling evidence that non-voters are so distinct from voters that they constitute a bloc ready to alter the fundamental balance of power in this country," wrote Rothenberg, editor of The Political Report, a newsletter that covers political campaigns.

He cited polling results that suggest that in Democratic years non-voters say they would have voted Democratic by even heavier margins than voters and that in Republican years the non-voters would have supported the Republican candidate by greater margins than voters did.

It has been more than a quarter-century since as much as 60 percent of the voting-age population voted. In the 1988 presidential election, the turnout of potential voters was 50.1 percent.

In an attempt to get non-voters into the system, the Senate Rules Committee on Wednesday approved a bill requiring states to let people register to vote when they apply for driver's licenses.

Similar legislation was approved by the House last year but faltered in the Senate under opposition from the Bush administration, key Republicans and election officials in several Western and Southern states.

But Rothenberg said his analysis suggested that recent presidential candidates Jimmy Carter, Ronald Reagan and George Bush would have won if every voting-age citizen had taken part in the election.

The differences between voters and non-voters have narrowed over the past 20 years, he said.

"The two groups are not yet identical, but they no longer constitute two very different-looking electorates."