New Jersey state Sen. Samuel Thompson said Monday he's leaving the Republican Party and plans to seek reelection this year as a Democrat.

Thompson, 87, said in a phone interview that he decided to leave the GOP after party leaders questioned his fitness for office because of his age.

"The betrayal by so many of my friends — that was too much for me," he said. "I am not leaving my party. My party leadership has left me."

WEST VIRGINIA STATE SENATOR LEAVES DEMOCRATIC PARTY FOR THE GOP

Thompson's departure would expand the Democrats' edge in the state Senate from 24 to 25 seats in the 40-member chamber.

Middlesex County Republican Chairman Rob Bengivenga said that although he considered Thompson a friend and mentor and wanted him to be part of the party's future, leaders decided to support another candidate for state Senate.

"I was hoping the Senator would embrace it and work together as a mentor and advisor for the years to come," he said in an emailed statement.

Sam Thompson

Republican New Jersey state Sen. Samuel Thompson switched parties after GOP leadership questioned his ability to serve. (AP Photo/Mel Evans, File)

Democratic Gov. Phil Murphy said in a statement that he welcomed Thompson to the party and called him a tireless public servant.

Thompson, a former research scientist who served in the Army, said he plans to run for office in this year's legislative election. He said unlike some lawmakers who work part time as legislators while also holding other jobs, he works only as a legislator.

"It is my life. It gives me something to do every day when I wake up to get there and help people. The alternative is sit at home and just wait to die? I have to be active," he said. "I think I’m doing a public service."

'WHAT AM I DOING IN THE DEMOCRATIC PARTY?': NY COUNCILMAN ANNOUNCES EXIT FROM DEMOCRATIC PARTY TO JOIN GOP

Thompson — a supporter of former President Donald Trump — said Trump's presidency was a success. Asked how he could fit in the Democratic Party with that view, Thompson paused a moment.

"Everybody is entitled to their own choices and what they’re going to do," he said.

Thompson represents the 12th Legislative District, which includes parts of Burlington, Middlesex, Monmouth and Ocean counties.

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All 120 seats in the Assembly and Senate are up for reelection this year. The primary is set for June, when Thompson could have faced a primary challenger against him in the GOP if he still ran. It’s unclear whether Democrats will run a candidate against him in the party's primary.