"I look at her and I see 'extreme carelessness.' I look at him and I see 'Miss Piggy.' I look at her and I see experience. I look at him and I see change," said Virginia Lopez Rey, summing up her dilemma as she tries to decide who to vote for in the upcoming US presidential election.

The ‘carelessness' reference refers to the FBI's characterization of Democratic Party presidential candidate Hillary Clinton's sometimes shoddy handling of classified information during her stint as secretary of state.

The 'Piggy' moniker is what Donald Trump hurled at a former beauty queen to fat-shame her after she gained weight.

Rey is a lawyer based in Miami Beach, Florida, who works in mediation. A Cuban emigre, she is strongly anti-abortion. In another year, pulling the lever for the Republican nominee would be easy for her.

Between a rock and a hard place

Travel 1,200 miles (1,900 kilometers) north, and Brooke Carpenter, a seventh grade English teacher in a suburb of Philadelphia, Pennsylvania, is facing a similar dilemma.

In the four presidential elections she has voted in, Carpenter has chosen Republican every time. She is an evangelical Christian who opposes abortion, favors lower taxes and worries that a left-of-center government would encroach on her religious freedom.

In a stream of consciousness she opens a window into her dilemma, which includes the Libertarian candidate Gary Johnson, only to then find herself trapped.

"At this point my leaning is Gary Johnson who is more pro-choice, but do you really want to vote for someone who realistically doesn't have a shot [at winning]," she said. "I really can't vote for Hillary Clinton and I really can't vote for Donald Trump so I guess [Johnson] is my option but is it a good option? I don't think so, he probably won't win. So are you throwing away a vote? I don't know."

Among the 10 or so swing states still up for grabs, Florida, with its 29 electoral votes is the biggest prize still to be claimed. Pennsylvania, with its 20 electoral votes, began tilting towards Clinton last month after a video tape emerged of Trump speaking about women in demonstrably vulgar and abusive terms.

Educated women are key

The Keystone state has not voted Republican in nearly 30 years, but Trump continues to campaign there in the hope enough white, working class, voters will head to the polls - and hand him the state.

Educated women have been a much-talked-about demographic in the presidential race, and those in swing states such as Florida and Pennsylvania will be especially important in determining the next US president.

But the cast of undecided voters is hardly a monolith this year, or any other year, according to election experts such as John Hudak of the Brookings Institute in Washington.

"They might be strong liberals, they might be supporting Bernie Sanders and don't like either candidate," Hudak said, referring to the self-described 'socialist' senator from Vermont, who surprised everyone with his unexpectedly strong showing in the Democratic primary race.

"They might be social conservatives," Hudak continued, "who look at Donald Trump and don't see the type of Republican they like, and certainly don't see that person in Hillary Clinton either, so they're all over the map really."

In addition to undecided voters, variously estimated to be between four and seven percent, there are also third-party supporters - those supporting Johnson, the Libertarian, or Green Party candidate Jill Stein.

Invariably third-party candidates lose support as the election nears, according to Sam Wang, a neuroscience professor at Princeton University and founder of the Princeton Election Consortium (PEC), a statistics-based website devoted to data on the US elections.

Many voters wanting to protest the political duopoly come to realize they are essentially throwing away their ballot by not voting for one of the two candidates that can actually win. Indeed, Johnson's overall support has gradually declined, from a high of 9.2 percent in mid-September to 4.6 at the beginning of November, according to an average of polls at RealClearPolitics.com.

Libertarian sees support evaporate

And nowhere has Johnson's support evaporated more than in critical swing states, such as Florida and Pennsylvania, where he has been polling just 2.4 and 3.0 percent, respectively. In states where Clinton has more comfortable leads, like Wisconsin and New Hampshire, support for Johnson has been comparatively strong, at 5.7 and 6.0 percent, respectively.

"Gary Johnson supporters split about equal between Hillary Clinton and Donald Trump," Wang said.

"If past history is any guide, most of those supporters will eventually vote for Trump or Clinton in the end," he continued. "So we can think of Johnson supporters splitting equally, like undecideds."

Stein, the Green Party candidate, had the support of just 2.1 percent of voters, just two-thirds of the support she had three months ago. If any more of them do decide to cast their vote elsewhere, Wang said it is clear where they will go.

"Stein supporters split pretty heavily for Hillary Clinton," he said.

Even though the number of undecided voters appears to be diminishing, trying to divide the election results based on undecided voters may ultimately be a fool's errand, according to Hudak, from the Brookings Institute.

"Clinton and Trump are very, very different candidates, and their differences are so obvious, even in policy," he said, "but there are still people who go into voting booths and make up their minds when they get in there."