The Massachusetts Democrat, one of Trump's strongest critics in the 2016 election, is a progressive hero who has made her name working to crack down on Wall Street greed and pushing for stronger financial regulations.
"The American people aren't dumb, @realDonaldTrump. We don't trust you with our nuclear codes AND we don't trust you with our economy," she tweeted in response to his Monday speech.
Trump's proposal would more than halve the number of income tax brackets and bring rates down to 12%, 25% and 33%. Trump proposed reducing federal income tax rates to 10%, 20% and 25%, which is a proposal that nonpartisan groups assessed would add trillions of dollars to the national debt.
If elected president, the Republican nominee would also lower the business tax rate for corporations and small businesses to 15%, down from the current top rate of 39%.
Warren denounced that specific move, saying it would mean there would be less money for education, infrastructure and research.
"And taxes? @realDonaldTrump wants billionaires & giant corps to pay EVEN LESS -- so there's less money for ed, infrastructure & research," she said.
In addition to tax cuts, Trump also proposed a moratorium on financial regulations, which conservatives say hinder economic growth.
"Cut regulations massively," @realDonaldTrump? No rules & no accountability means people get ripped off & markets blow up. Remember 2008?" Warren said. "A 'moratorium' on agency regulations means stopping new rules on shady payday lenders & too-big-to-fail banks, @realDonaldTrump."
"W/o rules, @realDonaldTrump, too many huge banks juiced profits by cheating ppl on mortgages, credit cards, etc, then crashed our economy," Warren said.
She also knocked Trump's recently announced economic team, saying ".@realDonaldTrump's econ plan was written by rich corporate insider advisors to make the economy even greater for rich corporate insiders."
She added: "Trump gets advice from Bear Stearns' fmr chief economist. So @realDonaldTrump: did BS need FEWER regs when they helped crash our economy?"
Republicans, meanwhile, praised Trump's plan.
House Speaker Paul Ryan's campaign spokesman, Zack Roday, praised Trump's proposal saying it "has many similarities to the job-creating plan at the heart of House Republican's Better Way agenda."
"A Better Way,"
is a plan Ryan introduced in June as speaker that lists changes to six policy areas, including tax reform.
Former House Speaker Newt Gingrich, who has endorsed Trump for president and was once considered as a vice presidential contender, called Trump's speech "a home run."
"Trump hit a home run today with his economic speech. This should end media complaints about lack if details. A vivid pro growth speech," tweeted Newt Gingrich, the former House speaker who Trump had considered tapping as his running mate.
He added: "Trump is for lower taxes.Clinton for higher taxes ($40,000 per American, $1.2 trillion). Mondale tried higher taxes, carried one state."
The Trump campaign did not immediately respond to a request for comment.