After President Donald Trump's reelection campaign used its massive email list to go after potential 2020 rival Sen. Bernie Sanders on two consecutive days this week, team Sanders welcomed the president's attacks as a sign that he views the Vermont senator as a serious threat to his hopes for a second term.

"The Trump campaign knows that Bernie is their biggest threat."
—Bill Neidhardt, Sanders' Iowa deputy state director.

In an email Wednesday attacking Sanders for condemning the likely illegal U.S. assassination of Iranian commander Qasem Soleimani, Trump described the senator as "Democrats' 2020 front-runner" and accused him of repeating "talking points straight from Iran' Ayatollahs."

"Bring it," Sanders foreign policy adviser Matt Duss tweeted in response to the email blast, which appeared to mark the first time of the 2020 cycle that Trump's campaign has used its list to hit the Vermont senator.

Following the Trump campaign's second email of the week—headlined, "FACT: Bernie Sanders Is A Wealthy, Fossil Fuel-Guzzling Millionaire"—Sanders said "Donald Trump is attacking us because he knows we will beat him in the general election."

Sanders staffers echoed the senator's response to the Trump campaign's attacks, which come less than a month ahead of the Feb. 3 Iowa caucuses.

"The Trump campaign knows that Bernie is their biggest threat," tweeted Bill Neidhardt, Sanders' Iowa deputy state director.

National polls and surveys of key states have consistently shown Sanders outperforming Trump in hypothetical general election match-ups. Sanders has also been surging in recent Democratic primary polls, as the senator's speechwriter David Sirota pointed out Thursday.

"It is not a coincidence that Trump suddenly began attacking Bernie the same week this national poll came out showing Bernie now in first place in the Democratic primary race," Sirota tweeted, citing a Reuters/Ipsos survey that put Sanders at the top of the Democratic field, two percentage points ahead of former Vice President Joe Biden.