– Legislative power brokers across the country have long designed district lines in backroom deals that entrenched their control for years, if not decades. But now, Democratic lawmakers in New Jersey are carrying out a power grab in an unusually public fashion: They are seeking to make Republicans a permanent minority by essentially writing gerrymandering into the state constitution.

The New Jersey plan comes amid a national reckoning over the consequences of gerrymandering and has been met by fierce opposition across the political landscape — and not just from Republicans and nonpartisan watchdog groups.

Even some national Democratic leaders have criticized the plan, fearing that it undercuts Democratic efforts to attack what they term Republican strong-arm tactics in state capitals across the country.

The proposal would amend the New Jersey Constitution, and New Jersey voters would need to approve it through a ballot measure.

It overhauls the makeup of a redistricting committee to give more power to legislative leaders. It also establishes a “fairness test” requiring district maps to reflect how major political parties perform in statewide elections for governor, senator and president.

In New Jersey, which has not elected a Republican senator since 1972 and where registered Democrats outnumber Republicans by nearly 1 million voters, that standard ensures that the redistricting process would begin on an uneven playing field. (New Jersey did elect a Republican governor in 2013, but the state has been trending Democratic.)

“It institutionally strips away the will of the voter,” said Tom Kean Jr., Republican leader in the Senate. “The will of the Republicans and unaffiliated voters in New Jersey would be ignored to the benefit of incumbent majority party legislators forever more.”

Stephen M. Sweeney, the Senate president, and Nicholas P. Scutari, a cosponsor of the bill, argue that the redistricting process is too often conducted behind closed doors by unelected officials and where deals are hashed out without any voter input.

“There’s nothing gerrymandering about it,” Scutari said. “If we have a significant advantage in voters, then you’re going to have a significant difference in legislative districts. If you took this matrix of guidelines and put it in Texas, you’d probably get significantly more legislative districts that favored Republicans.”