By Michael A. Memoli, Tribune Washington Bureau (TNS)

LONDONDERRY, N.H. — Catherine Johnson’s day started at 6 a.m. She left her home in Hanover, drove 100 miles southeast across New Hampshire to a campaign event in Plaistow, then worked her way back with stops in Londonderry, Bedford and Goffstown.

Her itinerary rivals that of some presidential candidates. But Johnson will be casting a ballot, not appearing on one. She wanted to do her homework.

“I’m having so much fun,” Johnson said recently as she talked of watching Republican Sen. John McCain and planning to see Gov. Chris Christie of New Jersey, who is seeking the Republican nomination. She also plans to attend a Democratic primary debate.

“I just want to vote for who I think is the best leader for this time in our country’s history. And I’m not sure I know who that is yet,” she said.

Johnson is registered as an independent — “undeclared” as such voters are called in New Hampshire — one of 380,993, more than 40 percent of the electorate, who can choose to vote in either the Democratic or Republican primary on Feb. 9.

She grew up in Republican politics, the daughter of a former state party chairman, and said she spent her 7th birthday stuffing envelopes for her father’s Senate campaign.

After voting for McCain in the 2008 primary, she supported President Barack Obama for re-election in 2012, she said. She met Hillary Clinton this year and is considering the former secretary of State, but is concerned about Donald Trump’s standing in the polls and considering which Republican might be the best alternative.

“You want your vote to count,” she said.

Not all undeclared voters will put in her kind of mileage in weighing their options, but neither is Johnson a total anomaly in this state, which grows obsessed with presidential politics every four years. Undeclared voters represent a significant wild card here, and campaigns will work overtime to monitor their changing attitudes in the final weeks before the first ballots are cast.

“You have to recognize there’s always going to be shifting ground because of the nature of New Hampshire,” said Joel Benenson, chief strategist for the Clinton campaign. “You have to be vigilant and staying on top of it, and looking for changes and asking as many questions as you can to assess who’s going to vote where.”

Many undeclared voters are not truly independents and vote consistently in one primary or the other, analysts stress. The true swing, independent vote here might be as little as 4 percent of the final electorate, said Andy Smith, a University of New Hampshire pollster.

But in a close primary contest, those voters can make a significant difference. So can undeclared voters who lean toward one party or the other but don’t vote in every election. Both groups add another unpredictable element to a state where more than a third of voters often make up their minds in the final three days before the primary, according to exit polls taken over the years.

In 2008, the last time both parties featured contested nomination battles, 75,522 undeclared voters chose a Republican ballot while 121,515 chose a Democratic ballot, according to Secretary of State Bill Gardner. In 2012, when President Obama faced no major opposition for renomination, 90 percent of the undeclared voters who participated in the primary pulled a Republican ballot.

Undeclared voters make up an even larger percentage of the electorate now than they did at the time of the 2008 primary. How — or whether — they choose to vote could have a big impact. Currently, both parties appear to have close races here. But if the Democratic race grows less contested, for example, some undeclared voters might decide to vote in the GOP contest instead.

Their potential effect is one reason why — in contrast with the Iowa caucuses, which tend to draw a narrower and more ideological electorate — candidates in New Hampshire “have to talk to real kitchen-table issues” with the broader electorate in mind, state Democratic Party Chairman Ray Buckley said.

McCain, a two-time winner of the New Hampshire primary, said candidates here face “two challenges: one, convince them to register with the Republican Party; and second, of course, is to select you.”

“It means that you can’t appeal to a narrow slice of the Republican electorate,” he added. “I think what it leads to, to be honest with you, is a little bit more centrist position on the issues.”

Barbara and John Opacki of Sullivan, in the western part of the state, are among the sort of voters McCain had in mind. They say they voted for Obama but have been disappointed in him recently, and are mainly considering Republican candidates.

“We like to look at both sides of the fence, to choose wisely,” John Opacki said after he and his wife attended a Christie town hall meeting in Peterborough.

They liked the New Jersey governor’s national security message and how Christie defended his post-Hurricane Sandy appearance with Obama just days before the 2012 election.

“Forget politics,” Barbara Opacki said. “They truly did work together.”

“We realize that he was way down in the polls,” John added. “But his message is really coming out nice and clear.”

©2015 Tribune Co. Distributed by Tribune Content Agency, LLC.

Photo: U.S. Senator John McCain (R-AZ) speaks during a campaign event for U.S. Republican presidential candidate Lindsey Graham (not seen) in New York July 20, 2015. REUTERS/Brendan McDermid