CEDAR RAPIDS, Iowa — For Senator Clinton, second choice matters.

One of the former first lady’s top obstacles heading into tonight’s Iowa caucuses could be the peculiar nature of the vote itself, in which supporters of a candidate who fails to reach a 15% threshold in each precinct must turn to another contender.

In a race where polls show the leading Democrats — Mrs. Clinton, Senator Obama of Illinois, and John Edwards — virtually tied, the top “second choice” of voters could make all the difference.

Surveys have consistently shown Mr. Edwards holding an edge in the second choice category, with one poll released this week by Insider Advantage giving him more than three times the support of Mrs. Clinton and Mr. Obama among voters asked whom they would pick if their preferred candidate did not reach the 15% mark.

Unlike a traditional primary or general election where people can vote throughout the day, the caucus requires voters to devote an entire evening at their local precinct and vote in public, not in secret.

The data has worried some backers of Mrs. Clinton, who has lost the lead she held in most polls earlier this year. “I think that could be the most dangerous thing for her,” a Clinton supporter, Linda Peterson, 63, said after attending the senator’s event yesterday here.

The potential impact of second-choice voters has spurred speculation of deal-making between top-tier candidates and those like Senator Biden of Delaware, Governor Richardson of New Mexico, and Senator Dodd of Connecticut. While those contenders are unlikely to reach the viability threshold in most if not all precincts, polls put their combined support in the double digits, meaning a significant pool of votes could be up for grabs.

Mr. Obama has explicitly asked Iowans to “make me your second choice” at some campaign events, and yesterday, Rep. Dennis Kucinich issued a statement urging his supporters to flock to the Illinois senator as a back-up. The stridently anti-war Ohio congressman has barely competed in Iowa, and most surveys put him at 1% or less.

President Clinton even obliquely referenced the unusual dynamic when he stumped for his wife yesterday at a middle school in Dubuque. “Anybody that understands the Iowa caucus and the cut-off understands that one vote can make a big difference,” Mr. Clinton said.

The complicated caucus process has led many Iowans who are already studying the candidates closely to think even more deeply about their choice. “Practically everybody I’ve talked to says they’ve lined them all up,” Lois Grove, 65, said after seeing Mr. Obama speak in Council Bluffs on Tuesday night. Mrs. Grove, a retired pastor and teacher, said she was giving Mr. Richardson a close look but had not made a final decision. “To pick one now I’m going to go to a dart board,” she said.

Mrs. Grove’s husband, Bill, said he would caucus for Mr. Richardson but was considering Messrs. Edwards or Obama as his second choice.

While many Iowans say they look at candidates’ electability when making their pick, the caucus rules allow voters who support long-shot candidates to give them a risk-free try before settling on one of the leading contenders. “Generally, I try to go with my ‘principle’ first. After that, I just go up the line,” Mr. Grove said.

Despite the polls showing Mr. Edwards holding a second-choice advantage, the political overlaps between the top-tier and lower-tier contenders are varied. Messrs. Edwards and Richardson have each tried at various times to run to the left on the Iraq War, with the former North Carolina senator pushing Congress to take a hard line on funding, while Mr. Richardson has vowed to leave no residual troops behind in a withdrawal, unlike Mrs. Clinton and Mr. Obama.

Yet Messrs. Biden, Richardson, and Dodd have all emphasized their decades of experience in public life, a message that closely resembles Mrs. Clinton’s.

“If you believe that Biden voters are more ‘experience’ voters, then they might be more likely to go for Hillary,” Rep. Anthony Weiner, a New York congressman who traveled with Mrs. Clinton in Iowa yesterday, said. He also cited speculation that Mr. Richardson was angling for the vice presidency in a second Clinton administration as a reason for his supporters to decamp to her side.

The top Democrats all spent the last full day before the caucus barnstorming the state, repeating their stump speeches and dispatching surrogates to help make their case. Mrs. Clinton brought along her mother and daughter, while actors Mary Steenburgen and Ted Danson made a cameo appearance to introduce her before a church crowd in Indianola. Offering a more personal view of Mrs. Clinton, Ms. Steenburgen said their friendship began nearly 30 years ago in Arkansas. She praised Mrs. Clinton both as a mother and a confidante, saying she was “the kind of girlfriend that secrets are safe with.” The former first lady, Ms. Steenburgen added, also had a “raucous belly laugh.” “It’s a little bit dirty, actually,” she said to laughter.

Senator Clinton was set to make a surprise appearance on “The Late Show With David Letterman” last night in the comic’s first show since leaving the air because of the strike by the Writer’s Guild of America. Mr. Letterman produces his own show and struck a separate deal with writers, allowing Mrs. Clinton to appear without risking criticism from labor unions for crossing a picket line.

Mr. Edwards concluded a 36-hour, overnight tour of the state with a concert in Des Moines with John Mellencamp. “I’m still thinking a little bit. My brain’s still functioning a little bit,” a visibly drained Mr. Edwards told a packed room of supporters in Cedar Rapids last evening.