WASHINGTON (AP) — Elizabeth Warren announced Friday that she would expand public health insurance during her first 100 days in office, but wouldn’t push for passage of a “Medicare for All” program until the third year of her presidency, a timeline that acknowledges how tough it will be to shift to a system of government-run health care.

The Massachusetts Democratic senator released a health care transition plan that first vows to build on existing programs, including the Obama administration’s Affordable Care Act. Warren says she’ll then work with Congress to pass pieces of a universal coverage proposal more gradually, with the whole thing being ready “no later than” her third year in office.

Allowing more time underscores Warren’s — or any candidate’s — difficulty in delivering on government-run universal health coverage. Winning congressional approval would be a heavy lift, no matter which party holds majorities in the House and Senate.

“Every serious proposal for Medicare for All contemplates a significant transition period,” Warren wrote in an online post. “My plan will be completed in my first term. It includes dramatic actions to lower drug prices, a Medicare for All option available to everyone that is more generous than any plan proposed by any other presidential candidate, critical health system reforms to save money and save lives, and a full transition to Medicare for All.”

Even as she continued to praise Medicare for All, though, Friday’s announcement represented a move toward the political middle on an issue that has been one of the most important to voters in the Democratic primary — which begins Feb. 3 in Iowa.

“Warren is trying to thread a very tricky political needle here,” said Larry Levitt of the nonpartisan Kaiser Family Foundation. “Warren clearly still supports Medicare for All, but she is not putting all of her eggs in that basket.”

Warren had previously said she would offer more details on how to implement her health care policy, but she laid out for the first time exactly how it will take up to three years.