Experienced presidential strategists told Reuters that while they expect some other Democrats to eventually become top contenders, Biden’s strength is likely sustainable.

They cited two main factors: The massive Democratic field makes it difficult for any one candidate to stand out, and voters tend to be risk-averse when seeking a candidate to topple a sitting president.

In recent elections, early front-runners such as Democrat Al Gore, himself a former vice president, in 2000 and Republican Mitt Romney in 2012 largely kept a lock on the nomination, despite some bumps along the way.

In 2008, Democratic favorite Hillary Clinton was overtaken by Barack Obama to secure the nomination, but Obama took advantage of a relatively small field to steadily amass support as the top alternative to Clinton. When Clinton ran again in 2016, Sanders tried to do the same thing with a similarly small field and almost succeeded.