WASHINGTON — Senate Republicans derailed a Democratic “Buffett Rule” bill Monday forcing the nation’s top earners to pay at least 30 percent of their income in taxes, using the day before Americans’ taxes are due to defy President Barack Obama on one of his signature election-year issues.

By a near party-line 51-45 tally, senators voted to keep the bill alive, but fell nine votes short of the 60 needed to continue debating the measure.