Four years ago, John Brooks cast his ballot for Barack Obama, becoming one of the voters won over by his promise for changing Washington. This time, he had been undecided, but he said Mitt Romney made his decision easier by placing Representative Paul D. Ryan on the Republican ticket.
The choice pushed him back to President Obama, said Mr. Brooks, 43, a sales manager at a Chevrolet dealership, who said that until now he had thought Mr. Romney was really a moderate, and he had been open to voting for him before he picked a running mate whose views on the budget he found extreme.
“Romney and someone else could have been a great team,” Mr. Brooks said Wednesday, a day after meeting the president at a restaurant in Cedar Falls, Iowa, during a stop on Mr. Obama’s three-day bus tour of the state.
The selection of Mr. Ryan, Republican of Wisconsin, has energized the party’s base and brought fresh enthusiasm and bigger crowds to the Romney campaign. It also has awakened casual Democratic voters like Mr. Brooks who recoil at a Republican budget and tax policy they see as unfair to the middle class.
But the past several days have also brought evidence of a subtle but broad strategic shift by Mr. Romney, one that many Congressional Republicans are concerned could divert attention from job creation, the issue they think will work best for the party.
In the midst of an election in which few voters have not already taken sides, he is now running a campaign more focused on energizing an anti-Obama coalition than on trying to expand the universe of Romney voters with an argument that he is the most qualified economic steward.
Mr. Obama has been responding in kind, opening a deeply divisive period in the race in which firing up hard-core partisans is taking priority over trying to pursue relatively small numbers of undecided voters in the middle. This week has unfolded in a series of harsh exchanges between the candidates, with the president mocking his rival’s character, and Mr. Romney accusing Mr. Obama of disgracing the presidency by waging a “campaign of division and anger and hate.”
Persuasion, especially on the Republican side, has given way to partisan stimulation. A sharp focus on the economy is giving way to ideology and personality.
For months, Mr. Romney tried making inroads among voters who were disappointed with, but not necessarily angry at, Mr. Obama. Republican groups have run television advertisements taking a softer approach by telling voters to feel no guilt for not supporting Mr. Obama.
Yet after a summer of pummeling from Democrats, Mr. Romney is returning the scorching fire in hopes of expanding his support among Republicans who more than anything want to turn Mr. Obama out of office in November.
The traditional balance between motivating core supporters without scaring off independent voters may be different in this election cycle, strategists in both parties say, because such a small share of people are truly independent and potentially open to either candidate. A series of recent polls in six swing states showed that only 5 percent of voters were undecided and only about 1 in 10 likely voters who had chosen a candidate said they were open to switching.
At this point four years ago, more than 1 in 4 voters nationwide said they might change their minds.