In some states, one political party dominates even without a lift from gerrymandering. The most extreme examples are Hawaiʻi, Massachusetts and Rhode Island, all of which have legislatures controlled by Democrats.

Their control is so complete that in about two-thirds of legislative races in each state last fall, voters had just one ballot option: the Democratic candidate.

The percentage of races lacking major party opposition was 71 percent in Massachusetts, followed by Hawaiʻi at nearly 69 percent and Rhode Island at 68 percent, according to a mathematical analysis by The Associated Press . Most of the unopposed candidates in each state were Democrats.

The AP examined all U.S. House races and about 4,900 state House and Assembly seats up for election last year using a statistical method of calculating partisan advantage designed to flag cases of potential political gerrymandering. The U.S. Supreme Court is hearing arguments Tuesday on a pair of cases alleging unconstitutional political gerrymandering.

The "efficiency gap" test used by the AP showed a negligible advantage for Democrats in Massachusetts and in Hawaiʻi in the 2018 legislative races and a modest advantage for Democrats in Rhode Island. While gerrymandering could be a factor in some districts, experts say the Democratic Party's dominance and the Republican Party's weakness in those states explain why there are so many uncontested races.

In Massachusetts, Democrats hold wide majorities in the House and Senate, along with every congressional seat and every statewide office except governor and lieutenant governor.

They maintained their veto-proof numbers in both chambers last year. In the 160-member House, they won 127 seats to the GOP's 32 seats, with one independent. The 40-member Senate has just six Republicans. Not surprisingly, state Democratic Party Chairman Gus Bickford said the lack of contested races reflects the ability of Democratic candidates to listen to their constituents.

"In many cases, the widespread popularity of Democrats prevents Republicans from even recruiting candidates to run against them," he said in an email.

That doesn't mean Republicans haven't tried. In 2004, former Republican Gov. Mitt Romney helped recruit 131 Republican candidates for legislative seats and raised money for them. Every candidate lost.

Massachusetts Republican Party Chairman Jim Lyons, who took over the post in January, said he can't speak to what party leadership did in the past but said he is committed to helping GOP candidates and activists turn the tide on Beacon Hill.

"In 2019 and moving forward, my goal is for this party to contest and win as many races as possible," Lyon said in an email.

In Hawaii, Democrats' dominance goes back decades.

Last year, the Hawaiʻi Republican Party contested just 5 of 13 state Senate seats up for election and 17 of 51 House seats. The party focused on candidate quality instead of quantity as it targeted districts where it had the best chances of winning.

The party gained one seat in the state 25-member Senate, its only one. In the 51-member House, the party held steady with five seats.

Analysts say the large number of uncontested races depresses voter turnout, which is among the lowest in the nation. In the 2018 general election, 52.7 percent of Hawaii's registered voters cast ballots.

"Democracy needs an opposition party," said Colin Moore, a University of Hawaiʻi political science professor. "You need someone who has an incentive to make policy decisions more transparent, to force hard choices on the majority party, to not allow everything to operate behind closed doors, which is often what happens in a single-party state."

To encourage more people to vote, Hawaiʻi lawmakers have introduced several election reform bills this year. These include voting by mail, automatic voter registration and using ranked-choice voting in special elections and partisan primaries.

Rhode Island Democrats won all statewide and congressional offices in November as the number of Republicans in the 75-member state House of Representatives dropped from 11 to nine.

State Republican Party Chairman Brandon Bell ran for a House seat but lost to a Democrat. Bell said this week that Republicans have a hard time competing and getting candidates to run "in a dark blue state."

Republicans have to target districts where voters picked President Donald Trump in 2016, rather than trying to field a candidate everywhere, he said. Bell said he thinks the way political boundaries were shifted in Rhode Island in 2012 gives Democratic candidates an edge in some areas.

Democratic Rep. Joseph McNamara, chairman of the Rhode Island Democratic State Committee, said that's a "bunch of baloney." There are so many uncontested races because the Republican Party isn't well organized in Rhode Island and Democrats have candidates with a better message, McNamara said.

"It's the smallest state in the union. Divide it any way you want and we'd still have the same number of people in the statehouse," he said.

The good government group Common Cause Rhode Island is proposing a public financing system for candidates seeking a seat in the Rhode Island General Assembly because so many legislative elections are noncompetitive.