President Joe Biden said Tuesday that his re-election campaign was prompted in part by Donald Trump’s decision to run for president again and an effort to block his predecessor from reclaiming the White House.

“If Trump wasn’t running, I’m not sure I’d be running,” Biden said at a campaign event in Boston, adding that he “cannot let him win.”

Biden has faced criticism from within his party, including from Rep. Dean Phillips of Minnesota, who is mounting a last-minute primary challenge, that he is too old and should step aside to let a younger generation take the torch. But Biden has opted to run anyway, a move that has long been seen as driven by the former Republican president and a belief by Biden that he is the only one who can defeat Trump.

The remark came as Biden made several campaign stops in Boston, where he took a series of jabs at Trump.

At a campaign reception earlier in the day, Biden referred to the former president as “an election denier in chief,” adding that “Trump and his MAGA Republicans are hellbent on destroying American democracy.”

NBC News reported earlier that Biden’s re-election campaign is boosting its war room operation this week to include back-to-back rapid responses to Trump’s town hall with Fox News on Tuesday and the GOP debate Wednesday, the final one before the Iowa caucuses, as his campaign builds on its argument that the GOP presidential platform will be rooted in Trump’s “MAGA agenda.”

As the Israel-Hamas war unfolds, a national NBC News poll last month found Biden’s approval rating had dropped to 40% — the lowest level of his term. That poll also showed Biden trailing Trump among young voters ages 18 to 34. While Trump garnered support from 46% of these young voters, Biden got 42%, a reversal from past election results and past NBC News polls.