New Jersey has backed Democrats in every presidential election since 1992, but a new poll released on Tuesday shows that the state would be in play in a matchup between Hillary Clinton and Donald J. Trump — as long as Gov. Chris Christie is not on his ticket.

A poll from Monmouth University found that 38 percent of registered voters support Mrs. Clinton and 34 percent back Mr. Trump. The survey, which had a margin of error of plus or minus four percentage points, found that the remaining voters would either back a third-party candidate or that they remain undecided.

“Blue Jersey doesn’t appear quite so blue at this stage of the campaign, but we should keep in mind that neither major party candidate has fully locked in the support of their partisan bases,” said Patrick Murray, director of the Monmouth University Polling Institute.

When that happens, Mr. Murray suggested, it could benefit Mrs. Clinton because there are more Democrats than Republicans in New Jersey.