Before Maryland voters reach a ballot question on same-sex marriage or a high-dollar plan to expand casino gambling on November’s ballot, they will first encounter another emotionally charged but seemingly clear proposition:

Should illegal immigrants be allowed to pay reduced, in-state tuition rates at Maryland’s public colleges and universities?

Among the landmark referendums that could alter life in the Free State, however, the effect of Question 4 may be the least well understood. How many illegal immigrants will go to college under the law? And at what cost to taxpayers?

Will illegal immigrants overrun Maryland campuses, as opponents charge? Or as backers believe, will their numbers be barely noticed and their higher education ultimately become a benefit to the public?

With campaigns for and against beginning final pushes this weekend, the fate of Maryland’s version of the Dream Act will rest in part on which side successfully fills in the blanks that helped push the measure to the ballot.

After Maryland’s legislature narrowly approved the measure last year, conservative lawmakers seized in part on its unknown costs to collect more than 100,000 petition signatures — twice as many as needed — to force the first referendum on a Maryland law in 20 years.

Since then, polls show that proponents have gained the upper hand in what, if passed, would be the first statewide “Dream”-related legislation approved by a popular vote.

They have marshaled a growing army of grass-roots advocates. Undocumented students who have attained high honors in Maryland high schools but then couldn’t afford to go on to college have revealed their identities at news conferences. Catholic, protestant, Muslim and Jewish clergy have joined to preach a common message that charity and parity in public college costs could pay dividends in the form of productive, tax-paying adult immigrants.

Proponents have built their lead while focusing little on numbers or costs, citing the only state study that said the measure most likely would not affect that many students or cost all that much — at least initially.

But proponents acknowledge that they have little faith in the state-generated numbers, and the potential range is vast: Undocumented immigrants could number in the hundreds, or in the thousands, at state schools within a few years.

Under Maryland’s Dream Act, students who can prove that they have attended Maryland high schools for at least three years and that either they or their guardians have filed state taxes would be allowed to enroll at community colleges at in-state rates.

Those who attain an associate’s degree or 60 credit hours could transfer to a four-year institution. At the University of Maryland, tuition is $7,175 for in-state students, compared with $25,554 for out-of-state students.

A nonpartisan fiscal analysis generated for members of the General Assembly before last year’s vote concluded there was little information available on the number of eligible illegal immigrant students, and therefore costs could not be “reliably estimated.”

State analysts nonetheless came up with a tally of “at least 366,” by dissecting budget numbers at Montgomery College, which does not inquire about the immigration status of its applicants but receives state aid for legal residents.

Without much explanation, analysts extrapolated that the number of illegal immigrant students would quadruple within two years, lifting the annual cost to the state from about $778,000 to $3.5 million.

The added cost to taxpayers would come from offsetting lower tuition rates for illegal immigrants at community colleges. Under the law, public universities would be on their own to cover lost revenue from fewer students paying higher out-of-state rates. Dream Act students, officials say, would not take slots away from in-state students.

A Washington Post review of trends in the dozen states that offer in-state rates for illegal immigrants also provided only vague guidance.

Maryland’s total number of undocumented students could, as in Kansas, remain minuscule for years. Or, as in California or Texas, it could rise closer to 1 percent of the state’s overall college population.

In Maryland, that scenario would put the total undocumented post-secondary population above 3,000.

What’s more, college officials in Texas, California and elsewhere said that it is too soon to tell whether illegal immigrants are enrolling at a higher rate this year in connection with President Obama’s decision in June to stop deporting some undocumented immigrants who were brought to the country as children.

In other words, has the potential for legal, future work status lured more illegal immigrants to pursue college degrees?

Kristin Ford, spokeswoman for Educating Maryland Kids, a coalition of labor unions, civil rights groups and religious groups backing the law, said the coalition believes Maryland’s experience will be similar to other states.

No state keeps clear records of its numbers of illegal immigrants who are receiving in-state tuition, in part to protect students’ identities, but none has also estimated that the number exceeds 1 percent of its college population.

“It’s certainly the case that no one knows for sure, but I think the fact that not a single college or university has come out opposed to this may hopefully calm [voters’] concerns,” Ford said, noting that Maryland’s law is also more stringent than most others in requiring immigrants to have filed state taxes.

But the effect of the changing federal landscape remains unknown.

In California, officials recently reported that nearly 10,000 illegal immigrants, or 10 times as many as expected, filed for the first year of private financial aid available for university students. Analysts there said it was too soon to draw conclusions about the number.

“I think it’s safe to say that it’s certainly going to cost taxpayers a lot more than Maryland projected, and the costs are going to increase exponentially, every year. That’s the message we’ll be taking to voters between now and November,” said Del. Neil C. Parrott (R-Washington County), who set up a Web site to help gather signatures to petition the measure to the referendum.

Parrott said the site is relaunching this weekend with fresh campaign material against the measure.

State Sen. Victor R. Ramirez (D-Prince George’s), who authored the legislation, countered that its lead in public polls suggests that Marylanders know what they’re getting and are in support.

“This is not a giveaway; this is an incentive for our residents, our neighbors, our friends. They want an opportunity, and if they are given one, they will give back to this country.”