RICHMOND — The secret plan began unfolding about two weeks ago. Senate Majority Leader Thomas K. Norment Jr. went to Lt. Gov. Bill Bolling with a way to redraw Senate districts and make them more favorable to Republicans.

But Bolling rejected the idea, fearing that it would set a bad precedent, according to two people familiar with the meeting but not authorized to discuss it publicly. Bolling, who would be needed to break a tie vote in the evenly divided Senate, also thought the move would so inflame partisan passions that lawmakers would lose sight of such priorities as transportation and education.

Republicans would have to wait for the right opportunity.

It presented itself on Inauguration Day, when Virginia Democrats basked in their second straight presidential win and one in particular traveled to Washington to witness President Obama’s swearing-in: Sen. Henry L. Marsh III (D-Richmond).

With the civil rights lawyer, who decades ago argued school desegregation cases and served as Richmond’s first black mayor, away in the District on Monday, Republicans saw their chance. They took up a bill that had been on the calendar for days, only to be passed over every time, and gave it the legislative equivalent of an extreme makeover.

Left over from last year, the original bill called for “technical adjustments” to House district boundaries. On Monday, without hearings or notice, Republicans amended it on the floor so that it also called for far-reaching changes to state Senate districts. The debate on the 36-page amendment was limited to 30 minutes.

The bill, approved 20 to 19, concentrates minority voters in a new Southside district and changes most district lines. Democrats said the new map would make eight districts, six of them held by Democrats, more heavily Republican. The map, which now goes before the Republican-controlled House, also puts senators R. Creigh Deeds (D-Bath) and Emmett W. Hanger Jr. (R-Augusta) into one district. It also adds more Democrats to three already deeply blue districts.

Gov. Robert F. McDonnell (R) was among those surprised by the new political map. He said he was concerned that his own party’s redistricting attempt will kill any bipartisan spirit and torpedo his ambitious agenda in his final year as governor.

McDonnell declined to say whether he would sign the legislation. He added that he was only informed of the Republicans’ plan shortly before the bill hit the floor Monday and watched it unfold on TV from his office in the Capitol.

“I certainly don’t think that’s a good way to do business,” said McDonnell, adding, “This was not an initiative that I advocated.”

Sen. John C. Watkins (R-Powhatan) said the changes would give Virginia a sixth majority-black district and protect the commonwealth from possible litigation under the Voting Rights Act. He also said the map would improve on the lines that Democrats had drawn to benefit their party when they controlled Richmond’s upper chamber.

“As those who were here then will recall, the 2011 redistricting process was not this body’s finest hour,” Watkins said. “The map that was produced was lambasted for dividing too many localities, splitting too many precincts, having high deviations between districts, violating basic standards of compactness and discounting communities of interest.”

In 2011, the then-Democratic-led Senate and the Republican-controlled House hatched plans meant to protect their incumbents, but McDonnell vetoed the plans saying the maps could split too many local jurisdictions and violate state and federal laws. The legislators then agreed to new maps.

Watkins said those lines still fell short, although the Republican moderate acknowledged in an interview Tuesday that he was somewhat uncomfortable leading the charge on the changes. He said he did so at the request of Senate Republican leaders.

Norment (R-James City) did not return messages seeking comment. On the Senate floor Tuesday, he said that the new map makes the districts “more compact,” noting that the Democrats’ plan in 2011 put him in a district “that ran me almost from the North Carolina line to the outskirts of Richmond.”

Democrats accused Republicans of trying to “pack and crack” black voting power and of exploiting the absence of a civil rights leader, on the occasion of the second inauguration of the nation’s first black president, to do it. That Monday also happened to be Martin Luther King Day only added to their anger.

“We’ve witnessed a redistricting bill thrust upon us without any notice . . . done under the guise of being good to black folk,” Sen. A. Donald McEachin (D-Henrico) said on the floor Tuesday.

The state constitution calls for redistricting every decade after the decennial census in years ending in 1. Democrats take that to mean that redistricting can only take place every 10 years, but some Republicans suggest that the 10 years is a minimum, not a maximum.

Legislators have tweaked the district lines in off years, sometimes at the behest of registrars who discover problems such as split precincts. But the map the Senate passed Monday, which would go into effect in 2015, went beyond the typical off-year tinkering.

Senate Minority Leader Richard L. Saslaw (D-Fairfax) compared the move to the sneak attack on Pearl Harbor.

“I realize the introduction of this particular amendment may be a surprise,” Watkins said as he addressed Senate colleagues.

“World War II was a surprise,” Saslaw shot back.

In a statement, U.S. Sens. Mark R. Warner and Tim Kaine also criticized the legislation.

“On a day when Americans celebrated Martin Luther King Jr.’s birthday and inaugurated Barack Obama as president, Virginia Senate Republicans took advantage of the absence of civil rights leader Sen. Henry Marsh to push through a hyper-partisan change to Virginia’s already gerrymandered legislative district map,” the Virginia Democrats’ statement read. “This is not the way we should be conducting the people’s business in Virginia.”

State Attorney General Ken Cuccinelli II (R), the presumptive Republican nominee for governor this year, said Tuesday that he had not seen the details of the redistricting plan.

“I have to deal with defending it now,” Cuccinelli said. “I’m not worried about the constitutionality of it. I’m worried about how it plays out in the General Assembly.”

Former Democratic National Committee chairman Terry McAuliffe, the lone Democrat running this year to succeed McDonnell, also weighed in.

“I join Governor McDonnell in urging the legislature to end this divisive partisan effort and instead focus on making Virginia the best place for business with mainstream solutions on transportation and education,” McAuliffe said. “We simply cannot afford to have the legislature spend more time on divisive partisan fights, especially as we have so many important issues to address.”

Bolling, who is weighing an independent bid for governor, had “grave concerns” about the Republicans’ plan partly because it would erase district lines adopted just two years ago. “He’s concerned that it could create a hyper-partisan atmosphere that could make it very difficult for us to address other important priorities,” said Ibbie Hedrick, deputy chief of staff for Bolling.