WASHINGTON—Paul Manafort, President Donald Trump’s former campaign chairman, faces an uphill battle in fighting the tax charges levied against him, former tax prosecutors say, given the extent of the evidence prosecutors have detailed against him and other recent developments in the case.

Special counsel Robert Mueller’s prosecutors have cleared what are often the biggest hurdles in offshore tax cases, experts said, including obtaining financial records from difficult jurisdictions including Cyprus and St. Vincent & the Grenadines. They also have emails from Mr. Manafort’s tax preparer that appear to show the preparer was misled, according to the indictment, and obtained the cooperation of Mr. Manafort’s longtime deputy, Richard Gates.

“It seems like there are lots of records, and the ones that are most difficult to get have been gotten,” said Kevin Sweeney, a former federal tax prosecutor who is now a lawyer at the firm Chamberlain Hrdlicka. “It sounds like there are probative emails, and that’s generally not a good thing. And there is a cooperating witness who is pretty pivotal to the whole thing.”

A spokesman for Mr. Manafort declined to comment, and his lawyers haven’t yet presented their defense in court filings. But Mr. Manafort has declared his innocence, challenged Mr. Mueller’s authority to bring the case, and said the indictments amounted to “untrue piled-up charges.”

Experts said potential avenues of defense include an assertion that the foreign income described in the indictment was actually loans that didn’t need to be reported, they obtained advice from accountants not referenced in the indictment, and that Mr. Manafort didn’t control the foreign accounts, as prosecutors alleged he did.