WASHINGTON (AP) — As President Joe Biden refocuses on his $3.5 trillion "build back better" agenda, the White House is preparing an urgent and aggressively populist-styled message for lawmakers and the American public: Whose side are you on?

In a memo being sent Tuesday to Capitol Hill and obtained by The Associated Press, the administration warns there is no time to waste in passing the package of corporate tax hikes and domestic initiatives by the end of the month.

"We face a fundamental choice in America right now as we rebuild our economy: this time, will everyone get in on the deal?" the memo says. "The time is now. We have to meet the needs in front of us. Not tomorrow, not months from now, not next year. Right now."

The urgent appeal comes at a crucial time, with House and Senate lawmakers assembling the package by a Sept. 15 deadline for drafting legislation. It also comes as Biden seeks to refocus on his core campaign promises after a brutal summer, punctuated by spiking COVID-19 cases and the deadly evacuations and withdrawal from Afghanistan.

Inside the White House, September is seen as a crucial month to make gains on Biden's agenda. In speeches and travel, the president is expected to take a more aggressively populist tone as he rallies support for the plan.

With work underway on legislation, Democrats have a precarious path to accomplishing Biden's big goals in the narrowly divided Congress. Essentially, all lawmakers in the House and Senate must be on board for the package to pass over unified Republican opposition.

But approval is not guaranteed as Democrats work to fundamentally reorder aspects of the U.S. economy and its governmental support systems. The legislative package aims to hike taxes on corporations and the wealthy to help Americans pay for child care, health care and many other domestic needs.

Democrats are already preparing for an onslaught of attacks over what Republicans call Biden's big tax and spending plans, as the GOP tries to wrest control of Congress by winning House and Senate seats in next year's midterm elections.

In framing the arguments ahead as a choice, the White House is showing congressional Democrats one way the administration plans to counter those Republican attacks.

"Republicans who oppose the Build Back Better agenda have chosen whose side they are on, too," said the memo from White House Communications Director Kate Bedingfield to House Democratic communicators.

"The Build Back Better agenda is about tackling those challenges, and leveling the playing field to ensure the wealthiest and corporations pay their fair share and working families get a fighting chance to succeed and prosper in this country," the memo said.