The number of independent voters in the state keeps climbing

Voting Ward 8 at the Graham and Parks School in Cambridge in November 2016.David L. Ryan/Globe Staff

If you’re a registered independent with Republican tendencies in a congressional district with 10 Democrats vying for the nomination, what do you do on Tuesday?

Do you stay true to your GOP leanings and pull a Republican ballot for the Third District, even if you find the contests a snooze? Or do you pick a Democratic ballot to record your honest preference — or to simply make mischief in a more compelling race?

Independents, who now make up the highest share of Massachusetts registered voters in at least 70 years, are the X factor in next week’s state primary.

That 55 percent of registered voters — about 2.5 million statewide — can choose the Republican, Democratic, or Libertarian primary ballots.

While the uptick in the share of unenrolled voters since 2016 is modest, the trend of more people not aligning with any party is poised to rapidly accelerate in the coming years as the state’s new automatic voter registration law will pull more people onto the rolls beginning in 2020.

The impact on Tuesday’s primary and the primary elections beyond: more uncertainty.

Boston-based pollster Steve Koczela said unenrolled voters historically make up the majority of Republican primary voters and around 40 percent of Democratic primary voters.

“Which ballot are independents going to choose in this election? That’s a great question,” he said.

Debra O’Malley, a spokeswoman for Secretary of State William F. Galvin, said many voters “choose to be unenrolled by default, as it is the safest option if you are not sure which primaries in which you will want to vote in the future.”

The current shares of registered Democrats and Republicans are plumbing the depths of the voter pool, state data show. The 465,952 registered Republicans account for just 10.4 percent of the state’s voters — the GOP’s lowest share since at least 1948.

Democrats are on a downward trajectory, too. The 1.5 million Democrats now make up 33.2 percent of voters on the rolls. That percentage hasn’t been this low since 1960, when Democrat Foster Furcolo was governor and his fellow party members made up just 30 percent of registered voters in Massachusetts.

Next week, for independents, there are draws on both sides of the aisle.

Statewide, three Republicans are battling for their party’s blessing to take on Senator Elizabeth Warren.

The race among state Representative Geoff Diehl, who has aligned himself with President Trump, and businesspeople Beth Lindstrom and John Kingston, who have framed themselves as more independent-minded conservatives, is expected to be a draw.

There are also competitive races on the Democratic side — from two high-profile contests for the US House of Representatives in the Boston-anchored Seventh District and the Lowell-centric Third District (where 10 candidates are slugging it out), to statewide secretary of state and gubernatorial battles.

Massachusetts political prognosticators said they were not sure how the mix of independent voters who choose a Democratic ballot may help or hurt individual Democratic candidates.

On the other side of the aisle, a GOP primary electorate with more unenrolled voters — who are seen as more likely to be moderate — may boost Lindstrom and Kingston, insiders said, while a higher percentage of registered Republicans in the primary might help Diehl.

Scott Lively, the Springfield pastor running a long-shot Republican primary campaign against Governor Charlie Baker, said the uncertainty of what each primary electorate will look like next week is “pretty wild, isn’t it? It’s really unpredictable at this point.”

Baker is widely expected to triumph over Lively, best known for his international opposition to what he calls “the gay agenda.”

Unenrolled voters can also pull a Libertarian Party ballot, although there are no contests on it this year.

Of course, just because a voter is not enrolled in either party doesn’t mean he or she isn’t aligned with Republicans or Democrats.

“A lot of independents actually lean to one party or another, so ‘independent’ is sort of a misnomer,” said Jeffrey M. Berry, a Tufts University political science professor. “But there are also calculating voters who choose the more interesting contest, and I would include myself in that category,” he said with a laugh.

Peter N. Ubertaccio, a Stonehill College political science professor, said the high percentage of unenrolled voters is among the reasons he expects a low turnout on Tuesday, the day after Labor Day.

“Even though you can vote in a primary as an independent, when a person makes the decision to register as unenrolled, they are indicating in some way a lack of a desire to be involved in party politics and that’s what a primary is at the end of the day,” he said. “There are several reasons for low turnout on Tuesday. This is one of them.”

Whatever the implications of the majority of Massachusetts voters eschewing party labels, the trend is poised to continue as a new automatic voter registration law goes into effect. It will put eligible voters on the rolls as not enrolled in any party unless they specify otherwise.

“It is likely that automatic registration would increase the number of unenrolled voters,” said O’Malley, from the secretary of state’s office. “It is also likely to increase the number of voters overall, which has historically added to the number of unenrolled voters.”

Pam Wilmot, who leads Common Cause Massachusetts and helped author the new voter registration law, said she expects somewhere in the vicinity of 500,000 new voters to go onto the rolls in the next decade, most of whom will not enroll with a party.

“The data from Oregon, a leader in automatic voter registration, show the vast majority do not pick a party because that requires an extra step — so most are unenrolled,” she said.