LYNN – City and town clerks urge voters swarming to the polls Tuesday to budget plenty of time for parking and, if necessary, standing in line.Polls open at 7 a.m. and close at 8 p.m. Secretary of State William Galvin predicts more than 3 million of the 4.2 million registered voters statewide will cast ballots. Four million residents were registered to vote in 2004 and 2.9 million voted.More than half (2,141,878) of Massachusetts’ registered voters are not enrolled as either a Democrat or Republican, although Democrats (1,559,464) outnumber Republicans (490,259) more than three to one.More than 120,000 voters have already cast absentee ballots, out of about 200,000 distributed, he said.Galvin said all voters should be prepared to show identification at polls to verify residency.Presidential candidates John McCain, Barack Obama and their vice presidential running mates are the main attractions in Tuesday’s election, but presidential candidates representing the Constitution, Libertarian, Green-Rainbow and Independent parties are also on the ballot.Libertarian Robert Underwood is running for the U.S. Senate along with Democrat incumbent John Kerry and Republican Jeffrey Beatty.U.S. Rep. John Tierney faces a challenge from West Newbury Republican Richard Baker and Republican John Cunningham of Revere is running against U.S. Rep. Edward Markey.In contrast to the choices available in the national election, local voters will decide relatively few contested legislative and county races.Democrat Lori Ehrlich faces Republican John Blaisdell in Swampscott, Marblehead and two East Lynn precincts.State Rep. Brad Jones, a North Reading Republican whose district includes precincts 1, 3 and 4 in Lynnfield, faces a challenge from Democrat Claire Paradiso of Reading.Voters will also decide three referendum questions Tuesday: One, eliminating the state income tax; Two, decriminalizing possession of small amounts of marijuana, and Three, phasing out greyhound racing by 2010.Galvin defended a new law signed by Gov. Deval Patrick last Wednesday allowing voters who moved within the last 18 months but had not re-registered to return to their old districts and vote on statewide and national races.The 11th hour change, which extended the original period from six months, sparked concern among municipal clerks, who said it would guarantee errors on Election Day and worsen lines and delays.