WASHINGTON _ Bernie Sanders and Elizabeth Warren are Senate colleagues, ideological partners and mutual admirers.
But their tacit alliance in the 2020 Democratic presidential primary is approaching a breaking point as they vie to become the progressive alternative to front-runner Joe Biden.
More than five months out from the Iowa caucuses, liberals are thrilled that two of their own are at the top of the field. But there's an underlying worry among some of their supporters that if both Sanders' and Warren's strength endures, they could clear the path for the more moderate Biden to win the nomination _ a scenario similar to how Republicans never united behind a single alternative to Donald Trump in the 2016 primary.