WASHINGTON (Reuters) - U.S. Senator Bernie Sanders on Tuesday said he is hiring a well-known liberal journalist with a history of sharply criticizing other Democratic presidential candidates, including Beto O’Rourke.

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Sanders’ campaign said it is bringing in David Sirota, whose work has appeared in The Guardian and Newsweek, among other outlets, as a senior adviser and speechwriter.

The move could stoke tensions within the Democratic field since Sirota has previously targeted O’Rourke, the former U.S. congressman from Texas who entered the presidential race last week, and other rivals.

Sirota on Twitter and in published articles has accused O’Rourke of siding with President Donald Trump and Republicans while a member of the House of Representatives, as well as being overly friendly with the oil and gas industry.

Sirota’s criticism of O’Rourke in December drew a warning from Neera Tanden, a top ally of former presidential candidate Hillary Clinton and president of the Center for American Progress, a Democratic think tank.

“A supporter of Bernie Sanders attacking a Democrat,” Tanden tweeted. “This is seriously dangerous. We know Trump is in the White House and attacking Dems is doing Trump’s bidding.”

The conflict was a reminder of the bad blood between the Clinton and Sanders camps when the two battled for the Democratic presidential nomination in 2016 and the mistrust between the party’s moderate and progressive wings.

Sirota also has slammed presidential candidates Cory Booker, Kirsten Gillibrand and Kamala Harris, as well as former U.S. Vice President Joe Biden, for being overly cozy with corporate interests.

Asked for comment, Sanders campaign spokeswoman Sarah Ford cited a HuffPost story last month about an email Sanders sent to campaign surrogates, asking them to “engage respectfully with our Democratic opponents ― talking about the issues we are fighting for, not about personalities or past grievances.”

Sirota worked for Sanders as a press secretary when Sanders was a member of the House of Representatives in the early 2000s.

An article Sirota wrote in 2013 for the liberal news website Salon praising the economic record of Hugo Chavez, Venezuela’s late socialist president, also attracted criticism on social media from Sanders’ Democratic Party critics and conservatives alike.

Sanders recently refused to label Venezuela’s current president, Nicolas Maduro, a dictator or recognize the opposition leader, Juan Guaido, as the country’s rightful leader - the current U.S. position.

Guaido invoked the constitution to assume the interim presidency in January, saying Maduro’s re-election was not legitimate.

O’Rourke said while campaigning in Iowa last week that he supported Guaido’s claim on Venezuela’s presidency.