Hillary Rodham Clinton spoke at a campaign event Tuesday at Mountain View College in Dallas to outline her vision for America.
The former Secretary of State launched a new offensive on taxes, saying Bernie Sanders' health care plan would require middle-class workers to pay higher taxes.
Clinton said Democrats shouldn't follow a proposal by "one of my opponents" that would eliminate the current system and turn it over to the states.
She said she was the only one at last weekend's Democratic debate who wouldn't raise taxes on middle-class families.
Sanders proposed a bill in the Senate in 2013 to create a single-payer health care system that would have increased income taxes and payroll taxes to pay for it.
The Vermont senator's campaign says his "Medicare for all" plan would reduce waste and save taxpayers money and Clinton backs a health system that "props up" campaign contributors.
Clinton won the endorsement of the Service Employees International Union on Tuesday, giving her the support of a labor powerhouse that backed President Barack Obama in 2008.
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The nation's largest health care union represents about 2 million nurses, health care workers and other caregivers and is among the most ethnically diverse unions in the country. The decision is a blow to Vermont Sen. Bernie Sanders, whose supporters had pushed against an endorsement.
"Hillary Clinton has proven she will fight, deliver and win for working families," said SEIU president Mary Kay Henry in a statement. "SEIU members and working families across America are part of a growing movement to build a better future for their families, and Hillary Clinton will support and stand with them."
It represents another show of strength for Clinton, who has locked up most of the major unions despite Sanders' message of helping workers overcome income inequality. In a statement, Clinton said she was "deeply honored" by the endorsement.
The third main Democratic candidate, former Maryland Gov. Martin O'Malley, is shifting staff from his Baltimore headquarters to Iowa and other early states as he struggles to raise money.
SEIU endorsed Obama over Clinton in early 2008, giving the future president a boost in the lengthy Democratic primary battle. Union officials said Clinton received a strong majority in the vote of its leadership and a recent poll of its membership found about 70 percent back Clinton.
Sanders spokesman Michael Briggs said the campaign was "gratified that hundreds of thousands of workers are part of the growing grassroots movement supporting Bernie's campaign to help working families."
The union has been at the forefront in the fight to get cities to adopt a $15 an hour minimum wage.
Clinton has endorsed raising the federal minimum wage to $12 an hour, a level below the $15 an hour that Sanders and former Maryland Gov. Martin O'Malley have sought. But the union says its support was about building a movement for higher wages, not about a candidate or a campaign.
The union could be an asset to Clinton in the general election because it has a large presence in several battleground states, including Florida and Colorado. Half of its members are women and about 40 percent are minorities, with many speaking languages such as Spanish, Chinese and Creole.
