The church's donation will likely help fund an existing advertising campaign. Archdiocesan officials have also sent materials to parishes and schools arguing against the ballot question.

The church's contribution represents about a 50 percent increase over what the antimarijuana Campaign for a Safe and Healthy Massachusetts has collected so far. The total, however, is still less than half of what has been raised by the referendum's proponents.

The Boston Archdiocese is pouring $850,000 into a last-minute effort to defeat a state ballot measure to legalize marijuana, calling increased drug use a threat to the Catholic Church's health and social-service programs.

"It reflects the fact that the archdiocese holds the matter among its highest priorities," archdiocese spokesman Terrence Donilon said of the donation. "It's a recognition that, if passed, the law would have significantly detrimental impacts on our parishes, our ministries."


Just last week, convening a group of interfaith leaders around strategies to defeat the measure, Cardinal Sean P. O'Malley said the archdiocese would spend only a small amount in opposition.

But "within the last few days," his thinking changed, Donilon said. "The more he thought about this and prayed about this, he thought this was the right thing to do because it directly impacts the people we're trying to help," he said.

Donilon said the money comes from a discretionary, unrestricted central ministry fund, not from parish collection baskets or other programs.

The prolegalization forces say they aim to create a regulated and taxed market, removing marijuana sales from the criminal sphere. They point to statistics showing that current marijuana laws disproportionately hurt people of color.

Opponents have cited the state's opioid addiction crisis, warning that marijuana is a "gateway drug" that leads to more dangerous habits. And they argue that "edibles" — candy-like products infused with cannabis – could lure children into trying the drug.


"The archdiocese has come up with a position that, frankly, we think is based on unfounded assumptions and junk science," said Jim Borghesani, a spokesman for the promarijuana group YES on 4. "But they can spend their money any way they wish."

Borghesani added, "What I think the archdiocese is missing is the terrible harm that (marijuana) prohibition has done to people of color, to people who have chosen a substance that is less dangerous than alcohol and have had their lives ruined because they've been arrested."

Voters on Nov. 8 will decide four statewide ballot questions. On marijuana — known as Question 4 — polls consistently show the legalization forces in the lead. A Suffolk University/Boston Globe poll released Thursday showed likely voters backing the proposal, 49 percent to 42 percent.

The marijuana referendum comes at a time when national opinion appears to be swinging starkly in favor of legalized recreational use. Four other states will vote on similar questions, with polls showing them also trending toward approval.

A recent Pew Research Center poll pegged national support for legalization at 57 percent, with 37 percent opposed — nearly the mirror image of public sentiment a decade ago. Increased acceptance in the states would likely ramp up pressure on the federal government to lift its decades-long ban.

Given the small window between the archdiocese's financial involvement and the election, and with the airwaves crowded with presidential ads, it remains unclear what impact the investment will have.


The church's recent record on ballot-question politics is mixed. In 2014, the state's four Catholic bishops joined other religious leaders in pushing a repeal of the law permitting casinos here, but did not raise enough money to air TV ads. That measure floundered, and casinos remained.

In 2012, the church helped lead the fight against a ballot measure that would have allowed doctor-assisted suicides. The Boston Archdiocese and its affiliated entities contributed about $2.5 million, and the proposal failed.

This year, Donilon said, the Archdiocese has identified the legalization campaign as a threat to its sprawling umbrella of services — from antihomelessness programs, to food pantries, to parochial schools.

"We provide extensive programs, and the church has historically spoken out on issues that are both a public policy matter and also impact the wider society in terms of serving those who are truly in need," Donilon said. "We're convinced now more than ever that these programs will take a negative impact. It's going to have a huge societal impact."

Much of the state's political establishment — including Governor Charlie Baker, Boston Mayor Martin J. Walsh, Attorney General Maura Healey, and House Speaker Robert DeLeo — has also spoken out against the measure.

The bishops' contribution represents the largest single donation against marijuana legalization aside from the $1 million check written by casino magnate Sheldon Adelson, a Dorchester native and conservative political financier. The antilegalization campaign has raised more than $2.6 million, including the archdiocese's contribution.


YES on 4 has raised $6.6 million so far, according to state campaign finance records.

Jim O'Sullivan can be reached at jim.osullivan@globe.com. Follow him on Twitter @JOSreports.