For nearly seven years, Kim Guadagno has largely stayed out of the spotlight as second in command to a nationally known governor.

But that has begun to change. Earlier this month, New Jersey's lieutenant governor said she wouldn't follow Gov. Christie's lead in supporting GOP presidential nominee Donald Trump.

Now, she and Christie are at odds over New Jersey's soon-to-kick-in gas tax increase. Guadagno, a potential 2017 gubernatorial candidate, took to the airwaves Thursday to oppose a ballot question backed by Christie that would constitutionally dedicate the new revenue to transportation funding.

Her comments during an interview on NJ 101.5 prompted a rare public rebuke from Christie's office, which declared that Guadagno's opposition to the constitutional amendment "must be a misunderstanding."

"The governor finds it hard to believe that the lieutenant governor supports giving an unguarded pot of money to the Democrat-controlled Legislature, rather than on needed infrastructure projects," Christie spokesman Brian Murray said in a statement — which was blasted out while Guadagno was still on the air Thursday.

A spokeswoman for Guadagno followed up with a statement doubling down on her position.

"Only by voting 'no' on ballot question 2 will we spark the debate we need to reform our system of taxing and spending, all while still allowing current projects to continue," Guadagno said in the statement.

Other evidence of friction between Christie and Guadagno has been laid bare by testimony in the George Washington Bridge lane-closure trial, now in its sixth week in Newark.

Former Christie aide Bridget Anne Kelly, one of the defendants, testified last week that the relationship was "a little tense."

"There were times where [Guadagno] was kind of put in a box and wasn't permitted to do a whole lot" by Christie, Kelly told jurors.

"She wasn't allowed to come to meetings with the governor," Kelly continued. "She wasn't allowed to see his schedule. It was very — it was a stressful relationship."

In addition to serving as secretary of state and overseeing business initiatives, Guadagno has frequently been designated New Jersey's acting governor — including Thursday, when Christie was traveling out of state, according to his office.

The governor's office didn't respond to a question about his whereabouts.

The public acknowledgment of a rift between the two comes as Christie's approval rating reaches new lows. A recent Fairleigh Dickinson University poll registered the governor's approval at 21 percent.

Earlier this year, Guadagno launched a privately funded 501(c)4 organization led by the operative who ran Christie's 2013 gubernatorial campaign. Guadagno described the organization, Building a Better New Jersey Together, as a "think tank."

"She's running for governor, and I guess she thinks she can distance herself from the governor," Senate President Stephen Sweeney (D., Gloucester) said at a news conference Thursday.

In the radio interview, Guadagno said she became opposed to the ballot question after a recent conversation with the radio show's host, Bill Spadea — a vehement opponent of the 23-cent-per-gallon gas tax increase, which takes effect Tuesday.

The question — passed unanimously in the Assembly, and with one no vote in the Senate — doesn't raise New Jersey's gas tax. Christie and lawmakers already did that when they agreed this year to increase it for the first time since 1988 to replenish the state's depleted Transportation Trust Fund. The deal also cuts other taxes, including the estate and sales taxes.

But Guadagno, who said previously that she opposed the gas tax increase, argued Thursday that voting down the ballot question "requires them all to go back to the drawing board."

"A vote for question number two is a vote for the gas tax," she said in the radio interview. Voting for the constitutional amendment, she said, "means we can borrow upwards of $12 billion."

The law increasing the gas tax says that "commencing on the day" the amendment takes effect, the authority "shall not issue transportation program bonds in excess of" $12 billion.

If the ballot question isn't approved by voters, "the bill will provide no increase in bonding capacity," according to a fiscal analysis from the Office of Legislative Services. Instead, the state would have to pay for transportation projects through its general fund, the analysis said.

Assembly Speaker Vincent Prieto (D., Hudson), said in a statement Thursday that the ballot question "has nothing whatsoever to do with the level of fuel taxation."

Voters who "support fiscal responsibility" should approve the question, Prieto said, calling the alternative "reckless policy."