WASHINGTON — Democrats in Virginia’s State Legislature took a blue-moon political step on Friday and voted to largely strip themselves of the power to draw new political maps next year — maps that could well have locked them into power for a decade. But to say they acted grudgingly is an understatement.

The State House of Delegates voted 54 to 46 to place a constitutional amendment on the November ballot that would establish an independent commission to draw maps in 2021, when population totals from this year’s census will form the basis for redistricting. Separate legislation laying out rules for the panel and its actions was sent to a conference committee and seems certain to be approved later.

Nine Democrats, who hold 55 seats in the 100-seat House, voted in favor of the amendment and 46 opposed it. Only support from the Republican minority ensured that the measure would go to the voters, who are expected to approve it.

That was a marked change from last year’s bipartisanship, when the Legislature approved an identical version of the proposed amendment, the first move in a two-vote process needed to place such measures on the ballot. At the time, Democrats were in the minority in the House, and the amendment passed with overwhelming support from both parties, 85 to 13. Republicans controlled both houses of the Legislature but were widely expected to lose power, which they did, in November.