Lucy Nicholson / Reuters Republican U.S. presidential candidate Donald Trump speaks at a campaign rally in Costa Mesa, California, U.S., April 28, 2016.

As Donald Trump moves closer to clinching the Republican presidential nomination, he has offered lavish praise for Bernie Sanders, who faces increasingly slim chances in his battle with Hillary Clinton in the Democratic race.

Trump has begun calling for Sanders to run as an independent if he does not win the Democratic nomination and said he may borrow talking points from Sanders’ speeches criticizing Clinton to use in a possible matchup with the former secretary of state in the Nov. 8 general election.

"He said some things about her that were so incredible – incredible – and so incredibly bad,” Trump said on Tuesday after sweeping five Northeastern primaries, adding Sanders had "been telling the truth."

Yet data and interviews with Sanders supporters suggest that winning over large numbers of them may be difficult for the New York billionaire businessman.

Even though Trump, 69, and Sanders, 74, a U.S. senator from Vermont, emphasize some common themes such as criticism of Wall Street and international trade agreements, there is only limited crossover appeal between the two candidates, according to Reuters/Ipsos data.

Among voters who back Trump, just 12 percent said Sanders would be their second choice if Trump were not in the race, only slightly higher than the 7 percent who said Clinton would be their next pick.

Sanders supporters were even less willing than Trump backers to consider crossing over. Just 8 percent of Sanders supporters said they would vote for Trump as their second choice, roughly the same as the portion of voters who listed Trump's Republican rivals Ted Cruz and John Kasich as their second choice.

Sanders supporter Joseph Hayes, 37, of Oregon, still has hope for his candidate. But if it comes down to Trump or Clinton, Hayes said it was an easy, if unpleasant, choice.

"I would have to vote for Hillary. Reluctantly so, but I would," he said.

People who outwardly back both Sanders and Trump are even more of a rarity. Donor rolls show just over two dozen voters willing to support both Trump and Sanders financially.

“I think Bernie Sanders is too poor to be bought, and I think Donald Trump is too rich to be bought by special interests,” said Royce Gourley, a real estate investor, who gave $2,500 to Sanders and $2,700 to Trump, Federal Election Commission filings show.

OUTSIDER APPEAL

But Trump could find an opening as both he and Sanders have gained strong followings among voters looking for an outsider candidate who will shake up the Washington establishment.

Both have made significant inroads among laborers and union members who support their opposition to U.S. trade agreements, including the Trans-Pacific Partnership, which critics say threaten U.S. jobs.

But the two are far apart on many issues, especially immigration. Trump has proposed building a wall on the U.S. border with Mexico and has proposed a temporary ban on Muslims seeking to enter the country.

Sanders has called such proposals "crap" and often criticizes what he calls "inhumane" deportation programs.

For Sanders supporters like Dave Berry, 62, of Tacoma, Washington, the wooing may pay off as voters weigh the decision to stick with their party or stir things up in Washington.

"I will probably put a check in Trump's column (in the general election)," Berry said of a possible Trump-Clinton general election contest.

He said he did not think the former reality TV star could win, but felt good about making Clinton uneasy about her prospects of securing an easy win.

One risk for Clinton is Sanders voters who may sit out the election or choose a third-party candidate if their favorite is not on the ballot.

Valerie Benson, 80, of Cleveland, said that if it came down to Trump against Clinton, "I don't know that I would vote for anybody then."

(Editing by Caren Bohan and Peter Cooney)