WASHINGTON — Senate Republicans on Monday blocked a move to open debate on the so-called Buffett Rule, ensuring that a measure pressed for months by President Obama and Senate Democrats to ensure that the superrich pay a tax rate of at least 30 percent will not come to a decisive vote.

But the fierce debate preceding the 51-45 vote — the Democrats were nine votes short of the 60 they needed — set off a week of political wrangling over taxes that both parties insist they are already winning.

Senate Democrats intend to return repeatedly to the legislation, named after the billionaire investor Warren Buffett, who has complained that he pays a lower effective tax rate than his secretary. On Thursday, House Republicans will counter with a proposed tax cut for businesses that they say would spur job creation but would cost the Treasury almost exactly what the Democrats’ tax increase would raise.

Republicans say they like that contrast, and their language ahead of the vote on a motion just to take up the Buffett Rule was harsh and aimed squarely at Mr. Obama, who first proposed a 30-percent tax rate floor for anyone earning at least $1 million a year last September. Senator Mitch McConnell of Kentucky, the Republican minority leader, went to the Senate floor and all but called Mr. Obama a liar.