Donald Trump has said the 2016 presidential election is “going to be the last election that the Republicans can win”, because if he is not victorious undocumented migrants “legalised” under a Hillary Clinton presidency will “be able to vote and once that happens you can forget it”.

“I think it’s going to be the last election that the Republicans can win,” he told the Christian Broadcasting Network on Friday. “If we don’t win this election, you’ll never see another Republican and you’ll have a whole different church structure. You’ll have a whole different supreme court structure.”

Trump has made immigration a central issue of his campaign, promising to build a wall on the southern border, to be paid for by Mexico, although Mexican leaders have rejected the idea. His promises to deport some 11 million undocumented migrants who live in the US have recently been thrown into doubt, however.

CBN host David Brody asked Trump whether he was referring to “what Michele Bachmann was talking about”, which was Clinton providing potential citizenship for “many of these illegals”, because “that means Texas and Florida could be gone”.

Bachmann, a former congresswoman from Minnesota who made a brief run for the White House in 2012 and is an adviser to the Trump campaign, last week told CBN: “If you look at the numbers of people who vote and who live in the country and who Barack Obama and Hillary Clinton want to bring in to the country, this is the last election when we even have a chance to vote for somebody who will stand up for godly moral principles. This is it.”

Both Texas and Florida have large populations of undocumented people, relative to other states, and the latter has been tightly contested in recent elections.

Trump repeated: “I think this will be the last election if I don’t win. I think this will be the last election that the Republicans have a chance of winning because you’re going to have people flowing across the border, you’re going to have illegal immigrants coming in and they’re going to be legalised and they’re going to be able to vote and once that all happens you can forget it.”

Two successive presidents, Democrat Barack Obama and Republican George W Bush, have failed to pass immigration reform plans through Congress that would have created very gradual paths to citizenship. Despite steady support by moderate Republicans for such plans, Trump has consistently rejected citizenship unless migrants leave and return to the US.

“You’re not going to have one Republican vote,” he said. “And it’s already a hard number. Already the path is much more difficult for the Republicans. You just have to look at the maps.”

In the Real Clear Politics head-to-head poll average, Clinton leads Trump 45.6% to 42.9%. The same site’s poll average for Florida puts the Democrat up by 44.8% to 44.5%. In Texas, Trump leads by 45.7% to 37.3%.

Data journalism site FiveThirtyEight has forecast that Clinton has a 71.3% chance of winning the White House. The website makes Florida the state most likely to tip the election, with an 18.7% chance. Solidly Republican Texas, it says, has a 0.7% chance of deciding who makes it to the White House.