As President Trump appears poised to capture the Republican nomination for his re-election bid, warning signs for his electability are emerging in opinion surveys, including a new Quinnipiac University poll that shows President Biden with a six-point lead over the GOP frontrunner in a head-to-head race. 

The Quinnipiac numbers follow the release of a Bloomberg/Morning Consult survey earlier Wednesday showing that a majority of independent voters in key swing states are unlikely to vote for Mr. Trump in November were he to be convicted of a crime or sentenced to a prison term.

That looms as a risk for the GOP front-runner in light of the federal and state indictments he faces in the courts. The national poll of registered voters puts Mr. Biden at 50 percent in a hypothetical matchup and Mr. Trump at 44 percent, the university said Wednesday. 

This marked a deterioration of Mr. Trump’s position compared with Quinnipiac’s December 20 survey, which showed a hypothetical matchup between the two would be “too close to call.” In that poll, Mr. Biden received support from 47 percent of survey respondents and Mr. Trump 46 percent.

Independent voters back Mr. Biden over Mr. Trump by 52 percent to 40 percent, the new poll from Quinnipiac finds, while the two candidates retain strong backing from their respective parties. Some 96 percent of Democrats support Mr. Biden and 91 percent of Republicans back Mr. Trump.

The Bloomberg News/Morning Consult survey released Wednesday found that while Mr. Trump currently leads Mr. Biden by six points in seven swing states that are likely to determine the outcome of the presidential race in November, a criminal conviction or a prison term for the GOP frontrunner could shift the dynamics significantly. 

The survey finds that majorities of voters in these swing states — Arizona, Georgia, Pennsylvania, Michigan, North Carolina, Wisconsin, and Nevada — would be “unwilling to vote for Trump if he is convicted of a crime,” as 53 percent of voters contend, or “sentenced to prison,”  as 55 percent of voters say, in one of the prosecutions he faces.

In what Bloomberg News calls “an alarming sign for Trump,” moreover, this shift is not only seen among undecided voters or backers of Mr. Biden. “Two in 10 voters who said they cast ballots for Trump in 2020 said they would not be willing to do so again in November if he’s found guilty,” Bloomberg says, and that “number is even higher for those who supported him in 2016, at 25 percent.”

The Quinnipiac survey notes a “widening” gender gap among voters it surveyed, with women backing Mr. Biden over Mr. Trump by 58 percent to 36 percent in a hypothetical rematch. Meanwhile men support Mr. Trump over Mr. Biden by 53 percent to 42 percent. In both cases, the margins are wider than when the same question was asked in a survey in December.

In the event the former United Nations ambassador, Nikki Haley, were to capture the GOP nomination and run against Mr. Biden, Quinnipiac found, she would have the support of 47 percent of voters versus 42 percent for Mr. Biden. Independents would break for Ms. Haley over Mr. Biden by 53 percent to 37 percent.

Quinnipiac also found that “a five-person hypothetical 2024 general election matchup that includes independent and Green Party candidates,” Mr. Biden would get 36 percent support and Ms. Haley would have 29 percent. In that scenario insurgent candidate Robert F. Kennedy Jr. would receive 21 percent, with Cornel West at 2 percent and Green Party nominee Jill Stein at 2 percent.