6:21 PM PT -- Trump just ripped prosecutors for convening a grand jury in his criminal probe ... he says the move is purely political.
In a statement, Trump says the probe is "being driven by highly partisan Democrat prosecutors" and he says it's just another witch hunt.
Trump adds ... "I will just have to keep on fighting like I have been for the last five years!"
Former President Donald Trump could soon be criminal defendant Trump ... prosecutors in New York have convened a grand jury that could indict him on criminal tax evasion and other charges related to the Trump org's business affairs.
The Manhattan district attorney recently convened the grand jury to hear evidence and weigh potential criminal charges against the former president ... according to the Washington Post. The grand jury will reportedly sit for 3 days a week over the next 6 months.
Prosecutors are said to be looking into Trump's business dealings before his 2016 election, including whether the Trump Organization manipulated the value of real estate properties to defraud insurance companies and banks.