(Reuters) - President Donald Trump’s campaign team has been developing a plan to portray White House hopeful Elizabeth Warren as dishonest and untrustworthy - based in part on her past claim of Native American ancestry - in a sign of recognition that she may become the Democratic nominee to face him in 2020.

FILE PHOTO: Senator Elizabeth Warren does an interview in the Spin Room after the fourth Democratic U.S. 2020 presidential election debate at Otterbein University in Westerville, Ohio October 15, 2019. REUTERS/Aaron Josefczyk/File Photo

For months, Trump’s campaign had focused its fire on Joe Biden, the early front-runner in the battle for the Democratic U.S. presidential nomination.

But Warren’s surge to the top of the Democratic pack in recent opinion polls and her strong fundraising - $9 million more than Biden in the third quarter alone - has not gone unnoticed, two people inside the Republican president’s re-election team told Reuters.

They said the campaign is evaluating the vulnerabilities of all the Democratic candidates and it was still too early to single out any of them as the likely nominee. But, one of the officials said, “Her rise is undeniable.”

The nascent game plan on how Trump would attack the liberal U.S. senator from Massachusetts differs from the manner in which his team had gone after Biden, seeking to portray the former vice president as corrupt based on allegations of wrongdoing - made without substantiation - arising from his son’s past role with a Ukrainian energy company.

The president’s team sees vulnerabilities for Warren among voters to the charge of being dishonest, based in part on internal campaign data, the members of Trump Victory - the name for his 2020 re-election campaign - told Reuters, asking not to be named while discussing internal deliberations.

They said the central pillars of any attack on Warren’s trustworthiness would be based on her assertion that she would not raise taxes on the middle class to pay for her ambitious Medicare for All healthcare system overhaul, and the controversy over her previous claims of Native American ancestry.

“This is the number one hit against Elizabeth Warren - her dishonesty,” said Rick Gorka, a spokesman for the Republican National Committee, which is working with the Trump campaign.

Gorka said any specific plan to go after Warren is in early stages, because the Democratic field remains crowded with 17 candidates still in the running.

Warren’s spokesman Chris Hayden declined to comment on the Trump campaign’s strategy of trying to frame the senator as untrustworthy. On the Medicare for All issue, Hayden noted that the cost structure of her proposal had been backed by renowned economists and experts.

MIT professor Simon Johnson, a former chief economist at the International Monetary Fund, and Mark Zandi, the chief economist at Moody’s Analytics, were among the economists who consulted on the proposal, her campaign said on Friday. Both economists signed a letter backing Warren’s funding calculations.

‘A SIMPLE TRUTH’

As some members of the Trump campaign’s staff watched the most recent Democratic candidate debate on Oct. 15 from inside their “war room” in Arlington, Virginia, they became increasingly animated to see Warren dodging questions from moderators and rivals on whether middle class taxes would rise under her healthcare plan, the campaign sources said.

Responding to critics, Warren on Friday unveiled a detailed proposal on how to pay for Medicare for All that she said would not require raising middle-class taxes even “one penny.”

It envisions new taxes on corporations and the wealthy to finance a government-run system that would ensure healthcare coverage for all Americans and ditch private medical insurance.

Warren’s proposal prompted the Biden campaign to say she was “hiding a simple truth from voters” that the plan would require higher taxes on the middle class to pay for it.

Warren, looking to remake the current costly healthcare system that is based on a patchwork of private insurance and public programs and has left tens of millions of Americans with no medical coverage, said her proposal would bring total U.S. healthcare costs to just under $52 trillion over 10 years.

“Elizabeth Warren continues to lie to every American and if her credibility wasn’t shot before, it doesn’t exist anymore after this $52 trillion fiasco,” Erin Perrine, a Trump campaign spokeswoman, said in a statement to Reuters.

Some Democrats scoffed at the idea of the Trump campaign attacking any opponent on the question of honesty.

Christopher Celeste, a Democratic donor from Ohio who has advised Democratic presidential candidates over multiple election cycles, said, “For Donald Trump to choose to attack somebody on honesty is truly the height of irony.”

Ben Wikler, chairman of the Wisconsin Democratic Party, added, “When Republicans chose Donald Trump as their standard bearer they took any chance to use honesty as a political argument and shot it into outer space.”

Perrine, the Trump campaign spokeswoman, told Reuters in response that “Donald Trump is the most transparent president in history.”

POCAHONTAS AND DNA TESTING

The two Trump campaign sources cited internal campaign data that shows concerns about Warren’s trustworthiness, based in part on her claims about her Native American heritage.

The issue has dogged her since her first campaign for the U.S. Senate in 2012, when Republican Scott Brown criticized her for being listed by Harvard University as a minority when she was a member of the faculty - based on her claim of Native American ancestry.

Warren has said that her Native American ancestry possibly goes back to the 1700s, including Cherokee blood on her mother’s side, according to family lore.

Trump, known for coining insulting nicknames for political opponents, has derisively called her “Pocahontas” - a Native American woman known for her involvement in the early 17th century with the English colonial settlement at Jamestown, Virginia. Warren has called it a racist taunt.

Last year, she released results of a DNA test that she initially said supported her assertion, even though it found only minimal Native American ancestry that dated back six to 10 generations.

That angered tribal leaders who said being a Native American is not determined by DNA alone but by membership in a tribe, and she has apologized several times since. Her Democratic rivals have steered clear of the issue.

The Trump campaign sources did not provide details of how they intend to attack her on the issue of honesty, as potential plans are still being formulated and they want to wait to see how the Democratic race unfolds.

Republican strategist Ford O’Connell, who worked on Republican John McCain’s 2008 presidential campaign, said he believes the Native American ancestry issue is not behind her.

“If you can undermine her credibility,” O’Connell said, “you can make the case that the policies she is promoting are not true either.”