At stake is the outcome of the highly charged presidential race, which could have an impact on the state's economy for years to come, along with ballot questions that have put Maryland at the center of broader debates over gay marriage and immigration.
"What we say could reverberate, really, across the country," said Todd Eberly, a political scientist at St. Mary's College. He predicted the passion surrounding this year's ballot initiatives will spark more referendums in the future. "This will be the new normal," he said.
Candidates and advocates made a final pitch to voters Monday — rallying in all corners of the state, knocking on doors in battleground neighborhoods and unleashing a flurry of last-minute phone calls designed to ensure voters will turn out. In making his own case in favor of the ballot questions, Gov. Martin O'Malley broke into song at an event in Baltimore.
Outside Maryland, President Barack Obama and Republican nominee Mitt Romney campaigned in six states. The incumbent and challenger closed by arguing they could do more to lead the country out of the economic doldrums that have marked Obama's first term.
"This nation is going to begin to change for the better tomorrow," Romney said at a rally in Florida.
"Our work is not yet done," Obama, his voice hoarse, told a crowd outside the Wisconsin Capitol.
Maryland officials were predicting a high turnout at the polls here — about 80 percent, deputy state elections administrator Ross Goldstein said. That is consistent with past presidential election years. More than 430,500 Marylanders had already cast ballots in early voting, nearly double the number who voted before Election Day in 2010.
In addition to the presidential race, Maryland will choose nine members of Congress, including a U.S. senator. Most of those races are not considered competitive, but the 6th Congressional District has captured national attention as Republican incumbent Rep. Roscoe G. Bartlett runs an uphill fight for re-election against Democrat John Delaney.
But it is the state ballot questions that will be a driving force behind today's turnout, Eberly and others said. Voters will decide whether Maryland will join six states and the District of Columbia in legalizing same-sex marriage.
Maryland voters also are weighing whether to allow a sixth casino, to be located in Prince George's County, and the addition of table games at all six gambling sites. They will decide whether some illegal immigrants can pay in-state tuition rates at Maryland colleges and universities. And they will determine whether the state keeps its new congressional map — designed to send a seventh Democrat to the House of Representatives — or if the squiggly-shaped districts are sent back to Annapolis for a do-over.
Individual campaigns have sprung up around each of those issues, emanating from church pulpits and television screens. Actor Brad Pitt, pop star Lady Gaga and New York City Mayor Michael Bloomberg have expressed support for Maryland's same-sex marriage law. Ravens center Matt Birk filmed a video last month stating his opposition. Opponents began a robocall campaign last week featuring Florida Republican Sen. Marco Rubio and 2008 presidential candidate Mike Huckabee.
Airwaves have been jammed with advertising over the gambling question, with the two sides spending more than $87 million in their effort to sway voters.
O'Malley, a Democrat, started his day Monday at a rally for expanding gambling. He stood with state lawmakers and repeated a theme supporters have used in some television commercials: A vote for Question 7 will keep Maryland gambling dollars at home rather than in West Virginia, which hosts a mega-casino.
In a twist on the John Denver classic "Take Me Home, Country Roads," O'Malley sang: "Maryland cash, bring it back — to the place where it belongs."
Opponents, who did not have any public events Monday, have built their campaign around skepticism that the new revenue from gambling will go to schools, as promised.
The governor then drove about half a mile to Federal Hill Park, where he stood with Sen. Ben Cardin — who is up for re-election — to pitch same-sex marriage.
Opponents took a lower-key approach but said they were plenty busy.
"We're running in a million different directions," said Alex X. Mooney, chairman of the Maryland Republican Party. The state GOP helped petition the same-sex marriage and the immigrant tuition measure to referendum, and are urging "no" votes on both.
"We would like to moderate the far-left bills from the Democratic monopoly General Assembly," Mooney said. The referendum process "provides a check and balance," he said.
Candidates in the Senate race engaged in their own last-minute campaigning Monday.
"Marylanders came out in record numbers during early voting, and I can tell that we will surpass records on Election Day, too," said Cardin, a Democrat. "You can just feel the excitement of folks who want to get out and cast their ballot because the stakes are so high for us and our children."
His opponents, Republican Daniel Bongino and independent Rob Sobhani, agreed.
"The response to our phone banks, canvassing operation and social media has been overwhelmingly supportive," Bongino said. "We are confident that we did everything in our power to win this election."
"The folks I've met will turn out to vote today," Sobhani said. "And I believe I have done my part in offering an alternative and a plan to put our state back to work."
The Associated Press contributed to this article.
10 things you need to know on Election Day
Have a last-minute voting question? We have answers from Maryland State Board of Elections officials Ross Goldstein, deputy state administrator, and Donna J. Duncan, director of the election management division. Here are 10 things you should know before heading to the polls:
Where do I vote?
Your local precinct. If you've misplaced the voter information card election officials mailed to you, look up your precinct online at the state Board of Elections website. Follow the "find your polling place" link.
2. What's on my ballot?
Federal, state and local issues created an unusually crowded the ballot this year. You can cast a vote for president, one of Maryland's U.S. senators and the representative for your congressional district. There are also seven statewide ballot questions, including whether to legalize same-sex marriage, expand gambling and allow some illegal immigrants to pay in-state college tuition rates. And there may be more local ballot issues unique to your area.
After you've found your polling place at the state's website, you can click on the link to see everything on your sample ballot. You can also print it out and take it with you into the voting booth.
3. How long will I have to wait in line?
It depends on when you go. Most years, the least-crowded window is between 10 a.m. and 2 p.m. Polls are open from 7 a.m. to 8 p.m. As long as you're in line by 8 p.m., you'll be allowed to cast a ballot.
4. What about my phone?
You'll have to turn it off when you go into the voting booth, so don't plan on checking your email or texting a friend if you forgot which candidate you liked. State laws forbid cameras or other means of communication within the voting areas. You're welcome to use your smartphone while you wait in line.
5. Does it matter if I go to the wrong precinct?
Yes. Election officials should be able to direct you to the correct location. Or you can choose to cast a provisional ballot, and elections officials will count as many races as possible. For example, if you go to a precinct that is not in your congressional district, your vote on statewide questions about president, senator and ballot initiatives will be counted but your vote for congressional representative will not.
6. Do I have to show a photo ID?
No. While photo identification is an issue in some states, Maryland does not have a mandatory voter-ID law. Except in a few circumstances, poll workers should not ask for identification. You may be asked for an ID if this is the first time you have voted in Maryland. If you are asked to show ID and do not have it or do not wish to show it, you can ask to cast a provisional ballot.
7. Does my boss have to give me time off to vote?
Only if you work a shift that gives you no time off between 7 a.m. and 8 p.m. In that case, state law requires employers to give up to two hours' paid time off for registered voters. The employers can also ask for proof you voted, which poll workers can provide if you ask. Those "I voted" stickers may not count.
8. If something seems wrong at the polls, what do I do?
First, try talk to the election judges at your precinct. If that does not resolve your problem, contact the election board in your jurisdiction or the state election board.
9. Can I bring kids to the polls?
Yes. Voters may bring up to two people under the age of 18 to the polling place provided the children do not disrupt voting.
10. I voted early. Can I change my vote?