By Richard Cowan and Doina Chiacu
WASHINGTON (Reuters) - U.S. Senator Kyrsten Sinema of Arizona is leaving the Democratic Party to become an independent, she said on Friday, just days after Democrats won a Senate race in Georgia and secured 51 seats in the 100-member chamber riven by deep political divisions.
"Like a lot of Arizonans, I have never fit perfectly in either national party," Sinema said in an article for the Arizona Republic newspaper.
An aide would not say whether Sinema would continue to caucus with Democrats.
Sinema herself, however, said she would not caucus with the Republican Party, according to an interview Politico published on Friday. If that holds, Democrats could still maintain greater governing control in the closely divided chamber, blunting the impact of her defection.
Sinema's surprise announcement came as the future of Democratic President Joe Biden's agenda in the second half of his term was already clouded by Republicans set to take control of the House of Representatives on Jan. 3.
With their strong conservative bent, House Republicans already have put Biden on notice they will seek deep domestic spending cuts and tougher border security steps. It will be up to Senate Democrats to foil Republican initiatives.
Sinema's statements so far indicate she will continue working in the independent-minded way she demonstrated over the past two years: collaborating with Democrats and Republicans to enact legislation, while unafraid of erecting roadblocks that frustrate the White House, Senate Majority Leader Chuck Schumer and other fellow Democrats nationally and in her home state.
Sinema informed Schumer of her decision on Thursday, a Democratic aide said.
"She asked me to keep her committee assignments and I agreed," Schumer said in a statement. He added that even with Sinema an independent, Democrats will hold majorities on committees and will exercise subpoena power and be able to clear nominees without time-consuming procedural votes to break Republican blockades.
Sinema and Democratic Senator Joe Manchin have kept Washington in suspense over the last two years as they repeatedly withheld needed votes for legislation sought by Biden.
Almost a year ago, Sinema and Manchin killed an attempt by other Senate Democrats to temporarily waive the "filibuster" rule, which requires a supermajority of 60 votes for most legislation to advance toward Senate passage.
That stymied Democrats' push, supported by Biden, for significant voting rights reforms.
With her close ties to the U.S. pharmaceutical industry, Sinema complicated Democrats' efforts to force lower prescription drug prices before finally settling on a narrow version of a bill that became law.
On tax policy, Sinema is more in line with Republicans, opposing Biden's moves to raise taxes on the wealthy and corporations.
At the same time, Sinema and Manchin have worked in a bipartisan way on high-profile bills, including one that will bring huge government investments to combat climate change.
Just this week, Sinema and Republican Senator Thom Tillis unveiled an immigration reform plan that is getting bipartisan attention in the Senate.
Democrats have held the Senate with a 50-50 majority, as Vice President Kamala Harris has the power to cast tie-breaking votes. U.S. Senator Raphael Warnock's victory in Tuesday's Georgia runoff election handed them their 51st seat.
Two senators - Bernie Sanders of Vermont and Maine's Angus King - are independents but caucus with Democrats.
Sinema will be up for re-election in 2024, and Democrats are likely to vie for her seat.
A possible Democratic challenger could be Representative Ruben Gallego, who issued a statement on Friday saying, "We need senators who will put Arizonans ahead of big drug companies and Wall Street bankers."