Democrats are planning their first major gesture of opposition to Donald Trump’s presidency, and Bernie Sanders is leading the charge.

Democratic members of Congress received a letter today calling on them to join the Vermont senator in organizing rallies across the country on January 15. The goal is to galvanize resistance to Donald Trump as Congress returns to work, just a week ahead of the next president’s inauguration.

The letter came from the party’s two top officials, Senator Chuck Schumer and Representative Nancy Pelosi, as well as Sanders, who joined the Democrat’s leadership committee for the first time ever this year.

Sanders, whose run for the Democratic presidential nomination this year emboldened the left wing of the Democratic party, has seen his cachet rise in a party looking for leadership following Hillary Clinton’s defeat in November. Amidst the recriminations and a messy race to head up the party’s national committee, it hasn’t been clear who would be the face of Democratic opposition to Trump. Now, we have an answer.

While Sanders’ harsh critique of the millionaire and billionaire class turned off professional Democrats who benefit from the global economy during his primary run, the election results have convinced party leaders that they need to better articulate how their economic policies will lead to broadly shared prosperity. Schumer, hailing from New York, is known as a voice for Wall Street in his party, but is also a pragmatist who once wrote a campaign book about how he sought to appeal to middle-class families on Long Island.

Both Schumer and Pelosi, prodigious fundraisers themselves, will have noted Sanders’ ability to raise money in large quantities from grassroots donors for progressive causes. A spokesperson told Quartz that left-over dollars from Sanders’s presidential campaign will go toward organizing the Jan. 15 rallies.

The content of the events will be opposition to Republican budget proposals, which in past years have contained large cuts to the public health insurance programs Medicare and Medicaid, as well as to Social Security, the US public pension system. Such budgets have been a hallmark of Republican speaker of the house Paul Ryan, and at least some of the cuts—particularly to Medicaid, which is for the poorest Americans—have been endorsed by Trump.

On the campaign trail earlier this year, Trump promised to protect Medicare and Social Security from reductions, but his cabinet appointments so far have featured proponents of cutting back. Democrats hope they can split the Republican party by emphasizing the divisions between Trump’s promises of big government initiatives and traditional GOP free market bromides.

Launching rallies across the country is reminiscent of past conservative efforts to drum up opposition to president Obama’s administration. Asked if Sanders sees an example in Republican leadership’s relentless refusal to work with Obama, Sanders’ spokesperson Michael Briggs said Sanders “hopes to work with President-elect Trump on those issues where they can find agreement, and that might include trade, it might include a desperately needed infrastructure repair program, but of course the key is in the details.”