WASHINGTON — Donald Trump leads the Republican presidential field in polls of GOP voters nationally and in most early-voting states, but some surveys may actually be understating his strength.

With Trump dominating political debates in both parties, gauging his true level of support has become a crucial puzzle.

A new study of survey data by Morning Consult, a polling and media firm, provides one piece of the solution, although many uncertainties remain.

The firm looked at an odd occurrence that has cropped up repeatedly this fall: Trump generally has done better in online polls than in surveys done by phone.

The firm conducted an experiment aimed at understanding why that happens and which polls are more accurate: online surveys that have tended to show Trump with support of nearly 4 in 10 GOP voters or the telephone surveys that have typically shown him with the backing of one-third or fewer.

ONLINE POLLS MORE ACCURATE?

Its results suggest that the higher figure probably provides the more accurate measure. Some significant number of Trump supporters, especially those with college educations, are “less likely to say that they support him when they’re talking to a live human” than when they are in the “anonymous environment” of an online survey, said the firm’s poing director, Kyle Dropp.

The study does not answer all the questions about how large Trump’s vote might be. Indeed, even the gap between online and telephone surveys it set out to understand could be changing.

The gap has narrowed significantly in surveys taken in the past few weeks, which could suggest that Republican groups that were reluctant to admit to backing Trump have become more willing to do so recently.

Another issue is that Trump’s support in pre-election surveys might not fully translate into actual votes.

He has not invested as heavily as some of his GOP rivals in building the kind of get-out-the-vote operation that candidates typically rely on, particularly in early voting states.

Some of the polls that show heavy support for Trump have also shown him doing better among self-identified independents who lean Republican than among regular GOP voters.

At least some of those independents may not be in the habit of voting in primaries and caucuses, which could make a robust turnout operation even more necessary.

On the other hand, a candidate of Trump’s level of celebrity may simply not need much of a get-out-the-vote operation. No one really knows.

Another complication is that most polls made public this year have been of people nationwide, not of voters in the states that actually hold the first primaries.

In Iowa, which will kick off the election season with party caucuses Feb. 1, Trump has slipped into second place, trailing Sen. Ted Cruz of Texas in the majority of recent polls.

In New Hampshire, which holds the first primary Feb. 9, Trump leads, but less dramatically than in national polls. In recent weeks, he has averaged a bit more than one-quarter of the vote there.

Still, the Morning Consult experiment sheds considerable light on an issue that has puzzled pollsters for months.