Bernie Sanders is set to endorse Hillary Clinton for president as early as next week, the New York Times reports.

The Vermont senator, credited with pushing Clinton further and further to the political left over the course of the primary season, is expected to offer his official support after he remarked in an interview Thursday with Al Hunt of Bloomberg Views, “We have got to do everything that we can to defeat Donald Trump and elect Hillary Clinton. I don’t honestly know how we would survive four years of a Donald Trump as president.”

Sanders’ pending endorsement comes as he and his supporters continue to lobby for a more  progressive Democratic Party platform ahead of the national convention later this month. Additionally, Sanders has pushed Clinton to adopt more transformative campaign proposals to win over younger and more progressive voters—a prime example being her newly-updated college affordability plan.

Tensions surrounding the endorsement have been high, but observers note that by holding out, Sanders has maintained leverage to successfully push for some of his most progressive ideas.

Sanders was reportedly booed Thursday during a private meeting with House Democrats who were angry at him for withholding his endorsement. The reaction came after he told them, “The goal isn’t to win elections. The goal is to transform America.” One lawmaker reportedly said he was “squandering” his movement.

However, as Washington Post blogger Greg Sargent noted in response, “if Sanders is squandering his movement, it is odd that he continues to rack up meaningful victories in the battle to transform the Democratic agenda, if not the country.”

Sargent writes:

A Democratic source told the Associated Press that the two are discussing plans to make their first campaign appearance together next week in New Hampshire, where the announcement may take place.

According to the Times, the expected endorsement “was partly the result of daily talks between Mrs. Clinton’s campaign manager, Robby Mook, and the Sanders campaign manager, Jeff Weaver, about bringing together the two rivals and advancing the policy priorities of Mr. Sanders.”

Sanders’ campaign, the Times reports, is also reportedly “eager” for Clinton to continue moving to the left on issues such as healthcare and the Trans Pacific Partnership (TPP), although her position on the pro-corporate trade deal is not expected to be a deal-breaker for the Vermont senator. As for healthcare, the Times writes, one possibility is for Clinton to make “a new commitment press Congress to add a ‘public option’ to the Affordable Care Act [ACA].”

During the campaign, Sanders supported replacing the ACA with single-payer healthcare.