IN A SNEAK ATTACK notable more for its deviousness than its strategic acuity, Virginia Republicans engineered a bald-faced power grab Monday that would radically redraw the state’s electoral map without so much as a “yea,” “nay” or “maybe” from anything so trivial as a voter.

Taking advantage of the absence of a single Democratic state senator, GOP lawmakers in Richmond rammed through a radically gerrymandered map designed to hand them control of the 40-member state Senate, which is now split evenly between the two parties.

They were able to do so, on a vote of 20 to 19, because one Democratic senator was in Washington to attend President Obama’s inauguration. Touche, Republicans! Count that as a new low for hyper-partisanship, dirty tricks and the unaccountable arrogance of power.

The Republican move was executed in the style of a putsch, arising from a conspiracy and with no warning, public input or debate. Pressing their momentary numerical superiority, GOP lawmakers amended a routine House bill by tacking on a new map, devised in secret. Although this is one of the most partisan redistricting plans in Virginia’s history, the Republicans then cut off debate after 30 minutes.

Fortunately, Gov. Robert F. McDonnell, a Republican who counts as a grown-up amid the sneaky schoolboys who dominate his party in Richmond, correctly saw the ruse as an invitation to political Armageddon — and a threat to his own legislative agenda. “I certainly don’t think that’s a good way to do business,” said the governor.

Mr. McDonnell should immediately announce his intention to veto the map. If the redistricting is allowed to stand, it would radically reshape politics in Richmond. Unlike the GOP-dominated House of Delegates, the Senate has been in Democratic hands or closely divided since 2007. The Republican gerrymander, which could deliver several seats to the GOP, would change that at a stroke.

That this change would take place at mid-decade, rather than immediately following the decennial census as is customary, is of no concern to the Republicans, whose eyes are fixed only on the 2015 legislative elections. In embracing the radical option, they discounted all consequences, including the hypocrisy of their chosen fig leaf.

State Sen. John C. Watkins (Chesterfield), the mastermind of the ruse, defended it on the grounds that it would likely yield one additional black state senator. That rationale was rich, given that Republicans have steadfastly blocked Democratic attempts to redraw the electoral map for Congress to create additional minority districts. (Although minorities constitute more than a third of Virginia’s population, just one of Virginia’s 11House members is a minority.)

In fact, Mr. Watkins’s proposed map, which is certain to clear the state House, is a classic example of packing minorities into a district — in this case, a newly drawn one in Virginia’s Southside — in order to dilute their influence, and that of Democrats generally, elsewhere.

Shame on the witless Democrats for not anticipating that Republicans, given the chance, would resort to dirty tricks. And shame on Republicans for continuing their campaign to transform the General Assembly into a nasty, underhanded clone of Congress.

Of course, Democrats are dead set on retaliating, and who can blame them? They could, for example, blow up everything by using their votes in the Senate to block Mr. McDonnell’s proposals to reform public education and transportation funding. They could also block passage of the budget.

That’s why Mr. McDonnell, who professes not to have known about his fellow Republicans’ subterfuge until it was carried out, should immediately announce his intent to veto the map. Failing to do so will only invite legislative paralysis, subvert his own proposals and inalterably stain his legacy. The sooner he acts, the better.