President Obama has been secretly meeting with Democrats who are expected to or have expressed interested in pursuing the party’s nomination for 2020 over the last few months, according to POLITICO.

Obama has met with Sen. Bernie Sanders, Sen. Elizabeth Warren, and his former Vice President Joe Biden, among others.

Former presidential candidate Sanders has been linked to the 2020 nomination for a while. A new Democratic National Convention rule, however, could hinder his campaign before it begins.

According to the rule, any person vying for the Democratic nomination has to be a registered Democrat, which Sanders isn’t; he’s a registered independent. (RELATED: Proposed DNC Rule Would Undercut Bernie Repeat In 2020)

Warren has become a popular choice among the public as a potential nominee. She and the president have fought relentlessly since he was sworn in.

Trump popularized the nickname “Pocahantas” for the senator, who claims she is of Native American ancestry. Despite the meetings with Obama, she stated in an interview that she “[will] not run for president.” (RELATED: Elizabeth Warren Says She Is Not Running For President)

Biden has been very indecisive about a potential bid for the nomination, saying he’ll wait until after the midterms to decide. He and the president have a very adversarial relationship, both saying they could win in a fight against the other. (RELATED: POLL: Who Would Win In A Fight: Joe Biden Or Donald Trump?)

The former president has also met with Deval Patrick, the former Massachusetts governor; Mitch Landrieu, the former New Orleans mayor; Jason Kander, the failed 2016 Missouri Senate candidate; and Pete Buttigieg, the mayor of South Bend, Indiana.

It’s unclear who will get the nomination but they will have to go up against President Trump come 2020.