Virginia Democrats asserted control of the evenly divided state Senate on Tuesday and organized as a majority, tipping the balance of power away from Republicans and casting a toxic pall over Capitol Square after the GOP vociferously registered its complaints.
Party leaders wasted no time, muscling through the plan on the same day Lynwood W. Lewis Jr. was sworn into office after the Norfolk Democrat had his narrow special election win affirmed by a recount Monday.
The plan relied on Democratic Lt. Gov. Ralph S. Northam using his tie-breaking vote to take effective control of a chamber now evenly divided, 20-20, between Republicans and Democrats.
It delivered one chamber of the state legislature to Gov. Terry McAuliffe’s party, providing the newly elected Democrat with some political protection as he pursues his agenda in the face of a House of Delegates overwhelmingly controlled by Republicans.
Mr. McAuliffe’s office did not respond to a request for comment on the developments.
The GOP had used former Republican Lt. Gov. Bill Bolling’s vote to create the rules of the chamber in 2012 — giving themselves control over crucial committees in which votes are taken about which bills will be advanced to the floor.
GOP leaders had insisted that the rules passed two years ago would remain in place until 2016, the expiration of the current Senate term, but Mr. Northam said Tuesday that those rules were effectively void.
Senate Republican Leader Thomas K. Norment Jr. of James City County parried with Mr. Northam for much of the afternoon but said after the vote that he knew the die had been cast. He said Democrats should have a better reason for changing the rules than simply to turn back what Republicans did in 2012.
“‘You guys did wrong so we’re going to do wrong, and that balances it out,’” Mr. Norment said. “Really. Really. I can tell you that from my perspective today, all the partisan politics aside, the institution of the Senate of Virginia should be embarrassed today.”
The practical effects of the changes mean that Democrats will now be able to set the legislative agenda in one half of the state Capitol and to quash any politically damaging or controversial bills passed by the Republican-controlled House before they reach the governor’s desk.
The new structure has them controlling nine of the 11 standing committees with Democratic chairmen on each one — a sudden and dramatic reversal of the previous committee assignments in which Republicans controlled 10 of 11 committees.
Another provision will also allow the chairman of the Rules Committee, now Sen. John S. Edwards of Roanoke, to send any legislation substantially changed by the House to the full Rules Committee, rather than its typical course of moving back to the Senate floor.
Republicans said the move was akin to handing Mr. Edwards a veto pen for House-passed legislation he didn’t like, but Senate Majority Leader Richard L. Saslaw hinted that the move was in response to a procedural shuffle employed by Republicans in 2011 to enact new regulations on abortion clinics. Mr. Saslaw, Fairfax Democrat, introduced an amendment that would not allow the procedure to be used on finance-related bills, like the state budget.
But Mr. Norment said his objection wasn’t just to the reshuffling of the committee assignments but the fact that there was no historical precedent for it ever having been done mid-term.
He also vowed not to repeat what Democrats did in 2012, when the body ended up in a lengthy deadlock over the two-year budget before Sen. Charles J. Colgan, Prince William Democrat, surprised even his own party by voting with Republicans to break the 20-20 tie.
Mr. Bolling had ruled he could not break ties on the budget.
Sen. A. Donald McEachin sued to block Mr. Bolling from using his tie-breaking vote to help Republicans organize the chamber, as Democrats did Tuesday. After a judge declined to intervene, the Henrico Democrat withdrew the lawsuit.
The Senate similarly was split 20-20 during the 1990s, and Donald S. Beyer Jr., the Democratic lieutenant governor, intended to use his tiebreaking vote in much the same way as Mr. Bolling. The plan was abandoned when Sen. Virgil H. Goode Jr. threatened to leave the Democratic Party if leaders refused to share power.