Hillary Clinton and Bernie Sanders clashed forcefully on the debate stage Wednesday, with the Democratic presidential rivals opening lines of attack on immigration in the aftermath of Sanders' stunning upset in Michigan.

Donald Trump, the Republican frontrunner, wants to deport millions.

Sanders was more blunt on disagreeing with Obama.

Clinton slammed Sanders, a self-described democratic socialist, for voting against an immigration reform bill in 2007 .

But Sanders shot back that Clinton had taken anti-immigrant positions in the 2000s, such as prohibiting the issuing of driver's licenses to illegal immigrants.

Clinton has nonetheless passed the half-way point in the race to the 2,383 delegates needed to win the party's presidential nomination, after she handily defeated Sanders in the southern Gulf state of Mississippi.

Sanders has shown remarkable resilience and the intensity of the debate suggested Clinton was taking his challenge seriously.

Tuesday's big loser was Florida Senator Marco Rubio. He has been seen by party luminaries as the best mainstream hope of derailing Trump, but he has performed dreadfully in several recent primary contests, including those Tuesday.

Clinton attacked Trump's rhetoric as "un-American".

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