State Sen. Kai Kahele plans to challenge U.S. Rep. Tulsi Gabbard for her congressional seat, sources have confirmed.

Gabbard just began her fourth term in Congress this month, making the timing of a challenge unusual. She isn’t up for re-election until 2020.

The challenge is a sign that Gabbard, who has enjoyed high popularity ratings since being elected to Congress in 2012, is showing signs of vulnerability.

Neither Kahele nor Gabbard returned calls from the Honolulu Star-Advertiser for comment.

Kahele, who represents Hilo, is expected to announce his plans Monday in Hilo. Kahele is chairman of the Senate Land and Water Committee and has served in the Senate since 2016. He’s the son of the late state Sen. Gil Kahele.

Kahele is also a combat veteran, a major in the Hawaii Air National Guard and a pilot for Hawaiian Airlines.

Gabbard last week announced that she is running for president and has since weathered criticism from some members of her own party at both the local and national levels.

U.S. Sen. Mazie Hirono suggested in an interview with MSNBC this week that she would not be supporting Gabbard’s presidential run.

Shortly before announcing her White House bid, Gabbard penned a piece in The Hill in which she accused Democratic U.S. Sens. Hirono, Kamala Harris and Dianne Feinstein of religious bigotry in their questioning of judicial nominees, angering some members of the Democratic Party.

On Thursday former Democratic National Committee Chairman Howard Dean told CNN that he thought all of the party’s candidates are qualified to be president except for Gabbard.

“I don’t think she knows what she’s doing, and I don’t think she … is qualified. She’s not qualified,” Dean said.

Dean referenced Gabbard’s trip to Syria where she met with President Bashar al-Assad and her past statements about gay people. “I don’t know what she thinks she’s doing,” he said.

Gabbard has also faced tough media coverage relating to her past advocacy against gay marriage since announcing her presidential bid. She released a video Thursday in which she apologized for her past statements and said her views on LGBT issues have changed, citing her track record of backing legislation supportive of LGBT issues since being elected to Congress.

But Michael Golojuch, chairman of the LGBT caucus of the Hawaii Democratic Party, said he wasn’t convinced that Gabbard had truly changed her personal views on homosexuality. “Her evolution doesn’t ring true,” he said.

Golojuch said he was glad to hear Kahele was challenging Gabbard.

“I think it would be amazing to have Kai in that seat,” he said.

The state Office of Elections has said that Gabbard can simultaneously run for president and her House seat. However, she could face political pressure to resign from Congress as her presidential campaign draws out.