Democrat Lynwood W. Lewis Jr. thanks supporters and staffers on Tuesday in Norfolk for their efforts to send him to Richmond to represent Virginia's 6th Senate District. (Stephen M. Katz/AP)

Democrats took control of the Virginia Senate on Tuesday with help from a newly installed senator, inflaming Republicans in a divided Capitol and ensuring bruising political battles in the coming weeks and months.

While Republicans had braced for a loss of influence after a special election tipped the chamber toward Democrats, they were floored by an obscure but highly consequential change to Senate rules that Democrats also muscled through. It gives the chairman of the Senate Rules Committee the right to single-handedly kill any Senate bills that have been “substantially” amended in the House.

The Democrats’ push for power plunged the chamber into heated debate for nearly five hours and prompted some of the Senate’s more understated members to decry it in the starkest possible terms.

“This is one of the most dangerous amendments I think I’ve ever seen,” said Sen. Walter A. Stosch (R-Henrico), referring to the powers conferred on the rules chairman.

Sen. Richard H. Stuart (R-Stafford) said: “Take us off the committees — that’s fine. You took me off of two. But don’t injure the voters of Virginia, and that’s what you’re doing with this amendment.”

More broadly, Democrats intend to use their new brawn to advance the prospects of Medicaid expansion and to bring back some bills killed in committees earlier in the General Assembly session, including measures that would ban discrimination against gays and make the children of illegal immigrants eligible for in-state tuition at Virginia colleges.

Just two weeks after Gov. Terry McAuliffe (D) took office with a promise of bipartisan cooperation, the Democrats’ maneuvering sets up the Assembly for a partisan showdown between House and Senate more akin to the workings of Washington than Richmond.

Democrats assured their colleagues across the aisle that they were not playing partisan hardball. They said giving the rules chairman the new veto power over bills would keep the GOP-dominated House from hijacking Senate bills — as in 2011, when the House took a Senate measure on hospital infection control and turned it into legislation mandating new building codes on abortion clinics. They said the authority to kill such bills would be wielded judiciously — and rarely.

“The sky isn’t falling,” said Sen. Richard L. Saslaw (D-Fairfax), now the Senate majority leader. “I’ve only seen one bill, one bill in 38 years, that falls into the category we’re talking about. So don’t think everything we disagree [on] with the House is going to wind up in there.”

Those assurances did little to comfort Republicans, already rankled by McAuliffe’s recent shift from bipartisan outreach to a hard push on Medicaid expansion and liberal social issues.

“In a day full of outrageous power grabs, the decision by Senate Democrats to transform the Rules Committee into a ‘super committee,’ and granting extraordinary powers to a single senator rates as most outrageous,” said Senate Republican Caucus Chairman Ryan T. McDougle (R-Hanover).

Drawing on parliamentary procedures dating to the days of Thomas Jefferson, as well as fresh memories of Republican power plays in Senate reorganization and redistricting, Democrats remade committees to give the party control over what legislation dies and what gets to the floor for a full vote.

Kicking Republicans off crucial panels, Democrats gave themselves chairmanships of every panel and majorities on all but two — a pair of relatively inconsequential ones that the GOP had stacked with Democrats for the past two years.

There was one concession to the much-vaunted “Virginia way,” made only at the insistence of Sen. Charles J. Colgan (D-Prince William), who strictly observes the chamber’s traditional civility. Colgan will share chairmanship of the Senate Finance Committee with Stosch, his Republican colleague. But the Senate also voted to strip Stosch of his Senate pro tempore title and gave it to Colgan.

Under rules that the Senate has operated under since it was last reorganized in 2012, senators can be removed from a committee only by a two-thirds vote. But Democrats determined that it took only a simple majority vote to scrap the rules and adopt new ones, which they did before proceeding to remake the committees.

Democrats pulled all that off with the assistance of Sen. Lynwood W. Lewis Jr. (Accomack), who won a recount by just 11 votes the day before over Republican businessman Wayne Coleman to fill the state Senate seat vacated this month by Lt. Gov. Ralph S. Northam (D).

By holding Northam’s seat for Democrats, Lewis kept the 20-20 Senate evenly split but allowed his party to assume a working majority. Power shifted to Democrats because Northam, who presides over the Senate, is allowed to break most tie votes. Under Northam’s predecessor, Republican Bill Bolling, the GOP held that tiebreaking power.

In early 2012, after elections for the full Senate, Republicans took control of the chamber with Bolling’s support. Democrats, seeking a power-sharing arrangement, argued then that the lieutenant governor was not allowed to vote in such organizational matters. They filed a lawsuit trying to block the move, but it failed.

Republicans contended that the structure established in 2012 was supposed to last for the full, four-year term. But Democrats said Republicans did not defer to traditional political calendars last year, when the GOP sprang an off-year Senate redistricting plan that eventually died in the House.

Democrats also noted that four new senators — representing 10 percent of the chamber — have joined the body since 2012, after two resignations, one retirement and one death.

Sen. A. Donald McEachin (Henrico) led the charge for Democrats and had some sharp exchanges with Sen. Thomas K. Norment Jr. (R-James City), who until the power change was Senate majority leader.

McEachin justified the changes with a reference to the Republican takeover, saying that “all sorts of precedents changed in 2012, and this is just the continuing evolution of precedent-changing events . . . as we become a more modern Senate.”

In reply, Norment asked caustically whether McEachin knew the difference between the words “evolutionary” and “revolutionary.”

McEachin responded that it was “in the eyes of the beholder.”

Norment later went on to grill McEachin over the most controversial action of the day — a new rule giving the rules chairman, Sen. John S. Edwards (D-Roanoke), veto power over any Senate bill that the House amends “substantially.”

Previously, amended bills returned to the Senate floor, where all 40 members decided whether to accept or reject the House’s changes. Under the new rule, legislation that is “substantially amended” will instead go to the rules chairman, who can then decide to kill it or to send it to the full Rules Committee for consideration.

Democrats said the provision was inspired by the 2011 House amendments that turned a hospital bill into an antiabortion measure.

The full Senate had the chance to vote on the amended bill and passed it, a move that enabled House Republicans to get past the Senate Education and Health Committee, which had regularly killed abortion-related legislation.

“This is about the integrity of our committee process,” McEachin said. “All this amendment does is say: ‘Take a time out. If there is a new policy issue raised that is substantially different, then send it to committee, in this case the Rules Committee, for disposition.’”

Norment argued that the change disenfranchises the House and gives the rules chairman too much power, given that the chairman alone will have the power to decide when a bill has been “substantially” modified.

He tried to pin McEachin down on the definition of “substantially.” The Democrat said that would be when an amended bill raises policy issues not contemplated by the original bill.

“Is it in the eyes of the beholder?” Norment shot back.

Norment proposed an amendment that would strike the element related to the Rules Committee, but it failed. Saslaw successfully amended the measure so that power would not apply to budget or revenue bills.