WASHINGTON—Senate Democrats this week are planning a vote to extend the nation's borrowing authority through 2014, the latest sign that the focus of the budget impasse is shifting toward preventing a U.S. debt default.

Democrats are still debating their opening bid, but the most likely scenario is to advance a debt-limit bill free of extraneous policy amendments, as President Barack Obama has demanded. The bill would put the need for the next debt-limit increase after next year's midterm elections.

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