Republicans' power in New Jersey just shrunk again.

In a rare and unexpected move, longtime Republican state Sen. Dawn Marie Addiego of Burlington County announced Monday she is switching parties and becoming a Democrat.

“As gridlock in Washington dominates the news, it has become increasingly clear that in order to effect change you have to be part of the discussion and not on the outside looking in,” Addiego, who has represented South Jersey’s 8th District since 2010, said in a statement she released on Facebook.

“The people of the 8th District did not elect me to be content in the role of loyal opposition," the 56-year-old attorney added.

The move bolsters Democrats' ever-increasing hold of New Jersey government, giving the party an even larger majority in the Senate, the upper house of the state Legislature.

Addiego singled out Democratic state Senate President Stephen Sweeney in her statement, saying she’s worked with with New Jersey’s most powerful sate lawmaker over the last year on fiscal issues. She said she’s been impressed with Sweeney’s “tireless work ethic and unparalleled success at delivering for the people of New Jersey and particularly South Jersey."

“I am utterly convinced, as he is, that New Jersey’s current fiscal mess cannot be fixed by putting our heads in the sand and pretending it does not exist,” Addiego said.

Addiego also argued that the national Republican Party has drifted from her belief system.

“My core values that originally drew me to the Republican Party have not changed, but the party which once echoed the vision of Ronald Reagan no longer exists," she said.

“Oil drilling off our coast and tax policy which unfairly penalizes New Jersey families are just a few examples of a National Republican Party that has lost its way,” Addiego added.

Addiego did not immediately return a message from NJ Advance Media seeking further comment.

The move makes sense politically. Democrats have been bolstered and Republicans have taken a hit in New Jersey in recent years, especially since the rise of Republican President Donald Trump, who is deeply unpopular in the blue Garden State, according to polls.

With Addiego’s switch, the Democratic Party now controls the state Senate with 26 members to Republicans' 14. That’s the GOP’s lowest number since 1981.

Democrats also control the state Assembly, the Legislature’s lower chamber, as well as the governor’s office, which went to Phil Murphy last January after eight years of Republican Chris Christie.

And the party holds both of the state’s U.S. Senate seats and 11 of its 12 U.S. House seats — the latter of which is the result of Democrats flipping four House seats in November’s midterm congressional elections.

Addiego’s home county of Burlington has grown more Democratic in recent years. She narrowly won re-election in 2017, and last year the county Democrat Andy Kim unseat former U.S. Rep. Tom MacArthur in the state’s 3rd congressional district.

Addiego’s former running mate, Assemblyman Ryan Peters, R-Burlington, heavily criticized her on Facebook, suggesting she made the move out of “self-preservation.”

“Dawn’s decision is one she will have to live with,” Peters wrote. “You can serve with principle in either party, but switching parties for self-preservation isn’t something I would ever do.”

Addiego’s decision leaves only one Republican woman left in the state Senate: Kristen Corrado of Passaic County.

State Senate Minority Leader Tom Kean Jr., R-Union — the chamber’s top Republican — lamented the move:

Here's my statement on Sen. Dawn Addiego's announcement. pic.twitter.com/MyVjVfwhza — Senator Tom Kean (@tomkean) January 28, 2019

State Republican Party Chairman Doug Steinhardt suggested local politics were at play:

.@NJGOP Chairman @DSteinhardtEsq: “South Jersey Democrats see the writing on the wall and know they are only a few months away from being down one more Senator in LD-1." https://t.co/P9nVAPmdeZ — New Jersey GOP (@NJGOP) January 28, 2019

Addiego’s decision could also be wrapped up in a Democratic Party civil war between Sweeney, D-Gloucester, and the more liberal Murphy, with whom the Senate president sometimes clashes, most notably on fiscal issues.

Addiego, a longtime moderate Republican, is likely to be part of a relatively moderate South Jersey voting bloc aligned with Sweeney, which could come in handy as another fight appears to be brewing between the Senate president and governor, over the next state budget.

That also means Addiego likely won’t have to stray too far from her past votes.

Sweeney says New Jersey is facing a financial crisis and is suggesting more cuts to public-worker pensions and health benefits to help save the state money.

Murphy, a major union supporter, has not lent his support and could seek more tax hikes instead to pay for pensions — something Sweeney says he’s against.

Sweeney told NJ Advance Media on Monday that Addiego is “a good friend" and will be “another vote” for the party.

“I know she’s not a liberal,” he said. “But she’s been moderate on a lot of social issues. It’s a good thing. ... She’ll bring a view and a voice that’ll strengthen our team."

Before serving in the Senate, Addiego also served in the Assembly and was a Burlington County freeholder and an Evesham councilwoman.

“I am looking forward to working with both parties, the governor and all legislators to make New Jersey a better place for our families,” Addiego said in her statement Monday.

Brent Johnson may be reached at bjohnson@njadvancemedia.com. Follow him on Twitter @johnsb01. Find NJ.com Politics on Facebook.

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