Barack Obama warned on Saturday that US progressives risk creating a “circular firing squad” at a time when prospective presidential candidates are competing fiercely against each other to run against Donald Trump.

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The former president was speaking in Berlin, at an Obama Foundation event.

“One of the things I do worry about sometimes among progressives in the United States,” he said, “maybe it’s true here as well, is a certain kind of rigidity where we say, ‘Uh, I’m sorry, this is how it’s going to be’ and then we start sometimes creating what’s called a ‘circular firing squad’, where you start shooting at your allies because one of them has strayed from purity on the issues.

“And when that happens, typically the overall effort and movement weakens.”

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Among Democrats, the field of prospective presidential nominees has swelled to nearly 20. All are eager to appeal to a party base pushed left in opposition to a hard-right president and motivated by success in the midterm elections.

Championed by progressive luminaries including the Vermont senator Bernie Sanders and New York congresswoman Alexandria Ocasio-Cortez, policy ideas such as Medicare for All and the Green New Deal have achieved rising prominence.

On either side of the aisle, party primaries are traditionally brutal affairs in which candidates are tested against rivals from other wings or factions. The first debates of the 2020 Democratic contest are months away but fierce fire is already being directed towards some more centrist candidates.

Obama’s former vice-president, Joe Biden, for example, has not yet entered the race but has nonetheless attracted attacks on his record over close to 50 years in national life, including previous stances on racial issues and women’s rights. He has also struggled to respond effectively to claims from a number of women that he made them physically uncomfortable.

Among other candidates, the former Texas congressman Beto O’Rourke has been attacked over a voting record on issues such as the environment and immigration which indicates a willingness to engage with Republicans.

Kamala Harris, the California senator, has attracted broadsides over her prosecutorial record before she entered national politics.

Republicans are of course seeking to stoke the flames of controversy themselves. Sarah Dolan, executive director of the America Rising political action committee, told the Guardian this weekend the group’s “mantra this cycle is really just to cause chaos, especially with how big the field is”.

Facebook Twitter Pinterest Barack Obama’s remarks in Berlin.

In Germany, Obama also discussed the virtues and drawbacks of political compromise. He advised his audience of young Europeans to “take some time to think in your own mind and continually refine and reflect, ‘What are my core principles?’

“Because the danger is if you don’t know what your principles are, that’s when you compromise your principles away.”

He added: “You can’t set up a system in which you don’t compromise on anything, but you also can’t operate in a system where you compromise on everything.”

Answering questions, Obama said progressives needed to think about remaining “true to our values and principles while recognising that in democracies … the only way we are going to be able to get things done is that we agree to a certain set of rules and part of those rules are that you never get 100% of what you want”.

He cited the Paris climate agreement as an example of an imperfect deal achieved under his presidency with the aim of building on it later. Trump has withdrawn the US from the deal.