Among those who have voted or plan to vote in a Democratic contest this year, Sanders has 49 percent support to Clinton's 48 percent, a Bloomberg Politics poll found.

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It's just the second national poll of the cycle finding Sanders ahead of Clinton, along with a Fox News poll in February that had him up by 3 points. Clinton has led several recent polls by double digits.

Clinton has won 1,223 pledged delegates, along with the support of 467 superdelegates, according to the Associated Press delegate tracker . Sanders has 946 delegates and 26 superdelegates.

But hundreds of superdelegates have not yet picked a side, and those that have are free to switch at any point before the official convention vote.

Sanders also does better in head-to-head match ups against the Republican presidential candidates, although both Democrats have strong numbers.

Clinton trails John Kasich by 4 points in a head-to-head matchup. Sanders leads the Ohio governor by 4 points.

Clinton, a former secretary of State, may benefit from a shift in focus to terrorism following the attacks in Brussels on Tuesday, the poll found.

She holds a 3-to-1 lead over Sanders as the candidate Democrats believe can best handle Islamic terrorism.

The survey of 311 Democratic primary voters has a margin of error of 5.6 percent, while the margin of error for the 815 likely general-election voter is 3.4 percent. The poll was conducted March 19–22.