MIAMI  If voters engineer the Congressional makeover that strategists in both parties now expect, the implications for governance over the next two years, and for America’s political future, remain a mystery. Ascendant Republicans will have to juggle the Tea Party’s determination to block President Obama’s agenda with centrist voters’ desire for the two parties to work together on jump-starting the economy.

“The looming victories for Republican candidates next Tuesday is not a validation of the Republican Party at all,” former Gov. Jeb Bush of Florida said in an interview. Instead, he argued, they would reflect “a repudiation of the massive overreach” by Mr. Obama and Democrats and “disgust with the political class” for its failure to cooperate and deliver results.

“It could create a middle ground,” Mr. Bush concluded. “Or it could create a dismemberment of our political parties.”

In this fractious environment, the Senate race in Florida may represent the best-case situation for Republicans. After insurgent conservative Marco Rubio overtook Gov. Charlie Crist for the Republican nomination and built a general election lead, top Democrats were reduced to trying vainly to persuade their own nominee, Representative Kendrick B. Meek, to abandon the race to help Mr. Crist’s independent candidacy stop Mr. Rubio.