President Joe Biden’s campaign is surveying voters on a potential matchup between vice president Kamala Harris and former president Donald Trump as pressure for the president to step aside builds.

The poll-testing suggests the campaign is exploring the vice president’s chances against the presumptive Republican nominee if she became the Democratic one, the New York Times reported on Thursday. The campaign’s analytics team was surveying voters this week, according to anonymous sources who spoke with the Times.

The report comes as Biden remains adamant that he will stay in the race. The president sent a letter to Congress on Monday saying he was “firmly committed to staying in this race” and that Democrats on Capitol Hill should “end” talk of his departure.

Polling of this alternative matchup has been inconclusive. An ABC/Washington Post/Ipsos poll released earlier this week found Harris leading Trump by two points with 49 percent to Trump’s 47 percent. But an Economist/YouGov poll showed Trump beating Harris by four points.

An internal Biden campaign memo reviewed by multiple sources reveals the Biden campaign sees winning Michigan, Pennsylvania, and Wisconsin as the clearest path to victory. But all three of these states are in the “Toss Up” category, and the Cook Political Report moved Arizona, Georgia, and Nevada to the “Lean Republican” category on Tuesday.

Biden is also slipping in national polls, and more Democrat lawmakers are calling on him to drop out of the race. The CPR polling average showed Biden trailing Trump by more than two points at 44.3 percent, and the RealClearPolitics average has him down more than three points.

The list of Democrats who have publicly told the president to step aside is growing. Representative Hillary Scholten (D., Mich.) became the eleventh Democratic member of Congress to publicly call on Biden to end his reelection bid, and Senator Peter Welch (D., Vt.) became the first senator to do so in a Washington Post op-ed released Wednesday evening.