TAMPA, Florida — Walt Ellis, who spent the waning days of summer registering voters in Florida, issued a crucial reminder to each new person he signed up: "Don't forget to show up on voting day -- and bring your driver's license."

In past years, it might have been enough for Ellis, a staunch supporter of US President Barack Obama, to focus his efforts on getting voters registered and out to the polls on Election Day.

But Florida, like a number of Republican-governed states across the United States in recent months, has introduced tough new voter ID rules and other regulations that opponents say could hinder Democrats' turnout on election day November 6.

"I just try to educate people about what the rules are, so they know in advance that they need an ID with a picture and an expiration date," said Ellis, 40, who owns a business installing home entertainment systems in Washington.

He drove 1,000 miles (1,600 kilometers) to Florida, a key swing state critical to win the White House, for the volunteer effort.

With less than two months to go before the election -- and with so few undecided voters in the battle between Obama and Republican hopeful Mitt Romney -- experts say the vote will turn largely on who can get more of their core supporters out to the polls.

But civil rights activists warn that new voting restrictions across the US, mostly in states with Republican legislatures or governors, could significantly suppress turnout.

The Brennan Center for Justice, a non-partisan group at the New York University School of Law, says five million Americans could be affected by the new restrictions on voting, which were adopted or are under consideration in more than 30 US states this year.

The leader of the National Association for the Advancement of Colored People this week slammed voter ID laws as a cynical attempt by conservatives to blunt the growing electoral might of minorities and young people -- groups that turned out in record numbers to help elect Obama in 2008.

"We have seen more states pass laws in the past year pushing more voters out of the ballot box than at any point in the last 100 years," NAACP director Ben Jealous told a radio interviewer in Charlotte, North Carolina.

Those who support voter ID laws, however, say they are needed to safeguard the integrity of the voting process.

"We have millions of people voting... across this country. Those millions of legal voters deserve to have their votes protected," said Pennsylvania State Representative Daryl Metcalfe in a recent media interview.

"Every legally cast vote should be protected from being undermined by the forces of corruption, and individuals have a responsibility under the new law to secure the ID they will need," he said last month.

Voter ID laws are particularly controversial because some say they hearken back to earlier attempts by some state governments to restrict ballot access, especially for African American voters in the decades before the civil rights movement.

US Attorney General Eric Holder in July likened the voter ID law in Texas to "poll taxes" put in place in the South after the end of slavery, laws which for decades created an economic barrier that prevented black Americans there from voting.

The Justice Department "will not allow political pretexts to disenfranchise American citizens of their most precious right," Holder vowed.

Some of the measures have been rolled back by the courts. Earlier this year, a judge found Wisconsin's strict new photo ID law to be unconstitutional, a ruling now under appeal. A federal court struck down a measure in Texas last month.

But a Pennsylvania judge has upheld that state's new voter ID requirement, despite a challenge from plaintiffs who said it disenfranchises minority and poor voters.

Fourteen-year-old Walter Ellis, who accompanied his dad on the voter registration trip to Florida, said he was surprised at the lack of public awareness about the new law in the state.

"It's good to see what we have done about informing people and getting people to see that this matters, but I think we still need to do a lot," he said.

His father, meanwhile, said that with the uncertain legal landscape and the closeness of the polls, he'll drive back to Florida a couple more times before November for more efforts to recruit and inform voters about the identification requirements.

"The more we do our part, the better we can help preserve people's protections and rights that are in danger of being wiped out," the elder Ellis said.