Even after winning approval of a measure calling on President Trump to end the war, they continue to call votes to force Republicans to answer for it.

For the 13th time this year, members of the Senate filed into the marbled chamber on Thursday to cast their votes on a Democratic measure calling on President Trump to halt military operations against Iran or seek authorization to continue waging war in the Middle East.

The resolution faltered when Republicans banded together to block it, surprising no one who has watched the same ritual unfold week after week as a conflict that was supposed to last four weeks has dragged into its sixth month.

It looked like an exercise in futility, not only because similar efforts have failed repeatedly — the vote was 50 to 49 on Thursday, similar to last week’s — but because one such resolution actually succeeded last month, winning approval in both the House and Senate only to have Mr. Trump ignore it completely.

Yet Democrats refuse to let the matter rest, viewing the successive votes as part of a broader political strategy to highlight the growing disconnect between voters — who have increasingly soured on the war — and Republican senators who continue to defer to Mr. Trump in backing it.

All but three Republicans — Senators Susan Collins of Maine, Lisa Murkowski of Alaska and Rand Paul of Kentucky — opposed moving forward with the measure to rein in the president’s war powers, which was offered this week by Senator Kirsten Gillibrand, Democrat of New York. A single Democrat, Senator John Fetterman of Pennsylvania, voted against the measure. He has consistently crossed party lines on the issues since the war began.

Democrats’ efforts to end hostilities not sanctioned by Congress appeared to have succeeded last month, when the Senate approved a war powers resolution, delivering a congressional rebuke after four Republicans crossed party lines to back the effort. But the concurrent resolution — which was also adopted in the House — does not carry the force of law, and the president has asserted that he has all the authority he needs to continue the war without a word from Congress.