WASHINGTON - Rushing toward their party conventions, the rival presidential campaigns are trying to invigorate core supporters while reaching out to a sliver of undecided voters who harbor doubts about President Barack Obama yet aren't sold on Republican Mitt Romney.

In the past week the campaigns have engaged in a vigorous debate over Medicare, pushing aside the economy and jobs, for the moment. Romney has charged Obama with running a campaign based on hatred, Obama has renewed a fight over Romney's tax returns, and the issue of government spending has blossomed again.

These are the August seeds that candidates are planting with fence-sitting voters even as the campaigns try to get backers excited in time for the conventions. Republicans will gather in Tampa, Fla., in a week, and Democrats will be in Charlotte, N.C., in the first days of September.

Romney's selection of Wisconsin Rep. Paul Ryan as a running mate gave supporters extra motivation beyond their deep-seated antipathy toward Obama. In calling out Obama as a divisive, angry campaigner, Romney stoked Republicans' dislike of the incumbent and tried to cut into an Obama advantage, his likability even among voters who take a dim view of his policies or his performance as president.

At one point, Romney urged Obama to "take your campaign of division and anger and hate back to Chicago." The Obama campaign said Romney's remarks were "unhinged."

And so it went.

Obama dug in on his populist arguments, casting Ryan as the epitome of wrong-headed budget policies that will benefit the rich over the middle class. His campaign challenged Romney to prove his assertion that he had not paid less than 13 percent in taxes over a 10-year period.

"They're asking you to pay more in your taxes, not to reduce the deficit, or grow jobs, or invest in education, but to give another $250,000 tax cut to people making $3 million a year or more," Obama said.

The tax rhetoric is a hit with the Obama faithful, but it is also designed to cast doubt among undecided voters who are seeking a reason to support Romney. Even as Romney tried to sour Obama's personal appeal, the president countered with images of marital affection as he campaigned with his wife, Michelle.

These undecided voters, called "the persuadables" by the campaigns, represent about 6 percent of the electorate, according to recent polls. About 19 percent of voters said there was a chance they could change their minds, according to a Washington Post/ABC News poll last month. That's compared with about 10 percent undecided and 25 percent who could still have changed at this point in 2008.

Romney pollster Neil Newhouse says the number of undecided voters is often larger in a presidential election without an incumbent, as in 2008. He said the burden is on Romney to win them over.

"Voters have made up their minds about Barack Obama," he said. "They are undecided about Mitt Romney."

Polls show that Republicans are more enthusiastic about the election than Democrats. Much of the GOP energy comes from a desire to defeat Obama, and some Democrats believe Obama needs to find a way to match that determination.

"One of the challenges is that the Republican base seems more motivated that the Democratic base. That's a big challenge for the president," said Doug Hattaway, a Democratic strategist who was a senior adviser to Hillary Rodham Clinton's presidential campaign in 2008.

At Romney's Boston headquarters, Ryan is seen as a running mate able to energize conservatives who may have doubted Romney's ideological passion, to articulate a fiscal message that resonates with undecided voters and to bring intangible attributes, such as making Romney himself a better candidate.

"They may be playing to their base, but this election is about persuading the middle class, working class Americans," said Obama pollster Joel Benenson.

The Obama camp has promoted Ryan's status as a leading light of the Republican Party to better link Romney to House budget proposals that Ryan wrote and that Obama has been criticizing for two years.

"We think this is a choice that we've been talking about all along," Benenson said.

Obama has begun to hit Romney hard on Medicare, tying his rival to Ryan's budget proposal to overhaul the health care plan for older people. The issue is particularly important in competitive states such as Florida, Iowa and Pennsylvania that have large elderly populations.

Seeking to drive older voters away from Romney, Obama has seized on Ryan's plan to shift future retirees into a system dominated by private insurance plans.

"The truth is, I think they know it's not a very popular idea," the president said Saturday while campaigning in New Hampshire. "You can tell that, given the fact that both of them have proposed to voucherize the Medicare system. I guess they figure the best defense is to try to go on offense."

But Romney and Ryan anticipated the Obama reaction and went after Obama's health care overhaul, which includes reductions in Medicare spending of $700 billion over 10 years. While those cuts come from health providers, not from benefits to seniors, Romney and Ryan are still portraying them as harmful to seniors.

The Romney team also says the timing of the Medicare debate benefits them.

"It's actually fortunate for us that it's being litigated in August as opposed to October," Newhouse said. "If it comes up again in October, it's not going to be new information."

The Romney campaign says calls for smaller government resonate with independent votes, particularly undecided suburban women. The GOP used the issue in 2010 when it regained control of the House. Indeed, shifting views among independents on the role of government were a big difference between the 2008 and 2010 electorates, according to exit polls.

In 2008, 43 percent of independents said government should do more to solve problems, while 49 percent said government is "doing too many things better left to businesses and individuals." In 2010, that split was 28 percent do more to 65 percent saying government is overreaching.

But some Republican strategists warn that a debate over government spending has risks. They say undecided voters are all for fiscal discipline if it's linked to economic growth, but they recoil if they perceive spending cuts as austerity measures that would affect them.

"The fiscal battle injected into the national discourse is really going to hinge on who can sell growth and who is selling austerity," said Republican pollster Wes Anderson, who is polling in several congressional districts in states that are competitive in the presidential contest.

AP Deputy Director of Polling Jennifer Agiesta and Associated Press writer Ricardo Alonso-Zaldivar contributed to this report.

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