There are growing signs that former Vice President Joe Biden sees Sen. Bernie Sanders, I-Vt., as his most formidable opponent for the Democrats’ presidential nomination, Politico is reporting.

Publicly, Biden tries to dismiss Sander’s attacks on him. But there are strong indications, he is taking his rival very seriously, Politico noted.

The website pointed out that Biden has sought the endorsement of community leaders in Iowa who supported Sanders four years ago. And Biden’s recent five-day bus tour in Iowa concentrated on the eastern part of the state. The area is considered the biggest regional battleground for the two candidates because of its large concentration of working-class voters.

“They have to start forcing Bernie to address some of his obvious challenges,” said Simon Rosenberg, founder of the New Democrat Network and a senior strategist for the Democratic Congressional Campaign Committee in 2018. “The gloves-off strategy didn’t work for (Hillary) Clinton, and it isn’t going to work this time either.”

Clinton barely beat Sanders in Iowa 49.9% to 49.6 in 2016.

Biden’s aides acknowledge they respect the level of excitement from Sanders’ supporters, according to Politico.

“What is the lesson from 2016?” said Biden senior adviser Anita Dunn. “It’s to not underestimate Bernie Sanders and his supporters. It’s a strong, broad base of support. And he is an indefatigable candidate.”

An NPR/PBS NewsHour/Marist Poll found last month that Biden and Sanders lead the Democrats’ race for president. Among Democrats and Democrat-leaning independents, Biden had 24% support and Sanders had 22%.