In some areas, they are becoming as much a part of U.S. elections as voters and precinct workers: poll watchers, the sometimes unofficial monitors who go to polling places with the idea of stopping fraudulent voting.
Poll watchers come in all political stripes - conservative Republicans, liberal Democrats, anti-fraud groups, labor unions and even international organizations.
With the presidential race between Democrat Barack Obama and Republican Mitt Romney extremely close, some groups are looking to poll monitors to ensure a fair election on Tuesday.
But some voting-rights advocates and others are questioning whether such monitors could become an intimidating presence that leads some people - namely minorities and the elderly - not to vote, and slows down the voting process for others.
Some of the observers are credentialed by local governments to monitor the election from inside polling sites and will be allowed to challenge the right of people to vote. Some are lawyers representing the presidential campaigns and their parties, looking out for any irregularities that could be cited in a legal challenge of election results.
Many more poll watchers are likely be outside voting precincts - possibly in large numbers - and could scare off some potential voters, civil-rights advocates say.
"You can be just as harmful outside as inside by creating disruptions," said Eric Marshall, of the Lawyers Committee on Civil Rights Under Law. "We shouldn't be having bullies creating disruptions or intimidating voters at the polling place."
Much of this kind of criticism has focused on conservative groups such as True the Vote, which says it is mobilizing up to 1 million volunteers to monitor polling places.
Catherine Engelbrecht, founder of the group, disputed the notion that monitors will intimidate voters. She echoed the sentiments of Republicans who have said that questionable voter registration efforts in heavily Democratic areas have made it increasingly necessary to monitor for potential voting fraud.
Engelbrecht said the group wants to safeguard the election from fraud by making sure that only legal citizens can vote and dead people are removed from voting rolls.
"We do know that each and every (monitor) is making a huge difference, and we are ... hopefully doing a better job of safeguarding the process," Engelbrecht said.
Whatever they think of poll watchers, both sides in the election want to make sure their supporters turn out to vote, and that voter fraud - which is rare, elections officials say - does not create an advantage for the other side.
States have different rules on who is allowed inside a polling area to monitor elections. Each party usually appoints as many poll watchers as each state allows - each political party and each candidate can have one watcher in each polling room in Florida, for example - and they are credentialed to observe the election from inside the poll site.
In some states a poll watcher, described as a "challenger" in some state laws, can contest whether a voter is eligible to vote or has already voted.
The practice of allowing people who may have received only basic training as poll watchers before Election Day to question a voter's eligibility concerns some legal analysts and others who say the monitors' activity could deny some people their right to vote by slowing down the process so much that some people get frustrated and leave before voting.
"We should leave determining who is eligible to vote to the professionals at the polling place," said Marshall, who manages the Election Protection coalition, a nonpartisan group of more than 100 community and legal organizations. It has a telephone hotline (1-866-OUR-VOTE) that is collecting tips on alleged voter intimidation before and on Election Day.
Democrats and rights groups said poll watchers in some states have been trained by partisan groups that want to prevent their political foes from voting, and that many of the efforts against voter fraud are really attempts to curb voting among minorities and others who tend to vote for Democrats.
They cite groups such as True the Vote, which has held training courses across the nation. On Tuesday, the group's monitoring program will include poll watchers who are credentialed to work inside polling sites as well as others who will stand outside.
Groups such as the AFL-CIO, the largest U.S. labor federation, which is part of Election Protection coalition, have said they are worried that large numbers of True the Vote volunteers outside a polling site could drive away some voters.
Election law specialist Rick Hasen was not so sure.
He said there were typically many more Republican poll watchers in Democratic areas than the opposite. But he questioned whether they had any impact on voting.
"A lot of these end up being more bark than bite," Hasen said. "My guess is there's going to be a lot less of this than some people are forecasting."
'MISINFORMATION' IN TRAINING
The training of some poll watchers has come under scrutiny since September, when a secretly recorded training session for Republican poll watchers in New Mexico became public.
The video, recorded and published by a liberal-leaning group called ProgressNowNM, showed a Republican Party leader telling monitors voters in New Mexico must present photo identification and may not use interpreters while voting, both of which are incorrect. The state's attorney general is investigating.
A similar training course sparked criticism in Wisconsin.
The Obama campaign's chief counsel, Robert Bauer, sent a letter to Wisconsin Attorney General J.B. Van Hollen, asking him to investigate the Romney campaign's training of poll monitors.
Bauer's letter cited a training manual for Republican poll watchers - obtained by the liberal group Think Progress - that misstates the types of documents acceptable to prove residency. The manual, which Think Progress posted on the Internet, also incorrectly states that an assistant helping a handicapped voter needs to have proof of residency.
In his letter, Bauer questioned why Wisconsin's Republican Party would tell poll watchers to identify themselves as a "concerned observer" on Election Day, rather than as a representative of the Republican Party.
"The misinformation communicated to poll watchers, the intent and effect of which is to mislead voters, cannot be dismissed as an isolated example of this tactic," Bauer wrote.
Romney officials called Bauer's letter a "transparently desperate political stunt" and said they stand by the accuracy of the training materials.
A spokesman for Wisconsin's Government Accountability Board told NBC News it objected to the training handbook and would raise the issue with the Romney campaign.
Think Progress also posted a Republican training video from Iowa that tells poll watchers to alert the Republican Party's legal team if they see a voter who is allowed to vote without an ID. But there is no voter ID requirement in Iowa.
The Iowa Secretary of State's Office said it had contacted the state director of the Romney campaign and made this clear.
STATES COOL TO INTERNATIONAL MONITORS
Besides the U.S. groups vying for attention and access to polling sites, groups of international election observers will also be at polling places on Tuesday.
The human rights office of the Vienna-based Organization for Security and Cooperation in Europe (OCSE) is sending 44 observers who will be in the four states with the largest number of Electoral College votes - California, Florida, New York and Texas. It also has a team of 13 analysts based in Washington.
In addition, about 100 parliamentarians from OSCE's 56 member nations will observe the election.
The group has run into resistance in Texas, Ohio and Iowa, where state officials have balked at allowing OSCE monitors access to polling places.
Texas Attorney General Greg Abbott, a Republican, has warned U.S. Secretary of State Hillary Clinton that OSCE monitors are not above Texas law. In a separate letter to the OSCE, Abbott warned that failure to follow state law at voting precincts could subject the monitors to criminal prosecution.
A spokesman of the OSCE's human rights arm, Thomas Ryder, said observers would obey all state laws - including those that prevent them from coming within a certain distance of the polls.