Yet more than a month into his second presidential bid, his response underscored, once again, his willingness to stand apart from a party he has not joined but hopes to helm.

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His reply on Ms. Pelosi’s bill also afforded him momentary distance from ideologically similar rivals, including Senator Elizabeth Warren of Massachusetts, who on Wednesday tweeted out her own support for the Affordable Care Act.

Mr. Sanders’s unflinching position is a reminder to voters that he is unabashedly left-wing and unlikely to worry about working within the confines of the existing system anytime soon. It also reflects a schism within the Democratic Party over the best way to recapture power in Washington: Should Democrats project big, bold policy ideas that could fundamentally alter the political structure but face long odds in a hyperpartisan Congress? Or should they present incremental measures that are more likely to appeal to the center and could succeed sooner?

Some candidates, including Ms. Warren, are hoping they can bridge the gap and do both. Others are betting that there is a following for within-the-system politics. Candidates like Senator Amy Klobuchar of Minnesota and Beto O’Rourke, the former congressman from Texas, have spent the early days of their campaigns highlighting their common-ground legislation with Republican colleagues, and former Vice President Joseph R. Biden Jr., expected to enter the race next month, wears his bipartisanship like a badge of honor.

It is a debate that has raged within the party since before the midterm elections last year, with some strategists suggesting that bold, disruptive ideas like the ones championed by Mr. Sanders electrify voters and spur them to the polls.