Barack Obama is making a fresh appeal to Hillary Clinton's supporters, part of a strategy to expand and dominate the general election battlefield and put his Republican rival on the defensive nationwide.

Polls show Clinton's primary election supporters are still very much up for grabs. Presumptive GOP nominee John McCain is actively targeting them -- but the Obama campaign is confident it will eventually win over the voter bases that carried Clinton in contests from January to June.

The first big step happened Thursday evening, as Obama and Clinton headed to Washington, D.C., to meet with dozens of her top donors at the Mayflower Hotel. The two senators then travel to Unity, N.H., for their first joint campaign stop Friday.

In a symbolic gesture at the Mayflower Hotel, Obama wrote a personal check for $4,600 to the Clinton campaign, for himself and his wife, Michelle. The maximum individual donation allowed by law is $2,300.

Obama finance chair Penny Pritzker also wrote a $4,600 check for herself and her husband. Clinton campaign chairman Terry McAuliffe had it in his pocket and showed it to reporters waiting outside.

Some Clinton donors had been frustrated that the Democratic presidential nominee-in-waiting had not done more to help pay off her more than $20 million in debt.

Clinton is doing her part to rally the base.

"Every issue you care about personally ... is really at risk," she told the National Association of Latino Elected and Appointed Officials on Thursday in Washington, D.C. "We cannot afford four more years of the same ... And therefore we have to be determined to chart a new course and we cannot do that without electing Barack Obama our next president."

Clinton's supporters are a key part of the strategy the Obama campaign is drafting to win the November election.

In a strategy session with reporters Wednesday, aides passed around an overview stating their candidate would compete in all 50 states. The plan stressed that the campaign is already up with a TV ad in 18 states, including several that are not typical Democratic targets.

But the plan also stressed the importance of traditional battlegrounds like Ohio, Florida and Pennsylvania -- all states Clinton won during the primary. And the plan highlighted the support the campaign expects to receive from some of Clinton's most loyal backers: women and Hispanics.

"I want (Clinton) campaigning as much as she can," Obama said Wednesday. "She was a terrific campaigner. She, I think, inspired millions of people. And so she can be an extraordinarily effective surrogate for me."

A set of polls released Thursday out of Quinnipiac University showed Obama winning over voters in key swing states.

The surveys showed Obama leading in the battleground states of Colorado, Michigan, Minnesota and Wisconsin, and leading big among independent voters.

The polls were conducted in conjunction with The Wall Street Journal and WashingtonPost.com.

John Kerry carried three of those states when he lost to President Bush in the 2004 election. But he did not carry Colorado. Obama's lead there suggests he may have some momentum behind his quest to carry his campaign into traditionally red territory.

The campaign also highlighted recent polling leads in states like Virginia, Missouri, Colorado, Ohio, Iowa and New Mexico -- all states where Obama is running his general election TV ad.

But not all surveys suggest Obama will sweep up ex-Clinton backers to carry the major battlegrounds and ride roughshod over McCain territory.

A new AP-Yahoo News poll found 53 percent of Democrats who favored Clinton for the nomination two months ago now back Obama.

That's an improvement from April, when only 40 percent of those supporters favored Obama over McCain.

But 23 percent of Clinton backers still picked the Republican over the Democrat for the November election. Sixteen percent were undecided, and 5 percent were for independent candidate Ralph Nader.

Most polls show Obama leading nationally by single digits, but the Gallup tracking poll showed the two candidates tied, at 44 percent each, for the second day in a row Thursday. They were tied at 45 percent each Wednesday. And Obama and McCain have traded the lead in recent polls out of Florida.

McCain downplayed the early polls Wednesday, including one that showed him down by double digits nationally.

"The first lesson I want you to draw is that people are really ... not gonna start focusing on the campaign until the conventions," he said. "So a lot of this polling data is pretty much ... ‘Who do you like?'

He noted that he isn't far behind in most polls. "That's good for this stage of the game," he said, "particularly considering the headwind we have on our economy."

Obama's campaign isn't just relying on Clinton herself. It also has sought help from prominent officials who backed her, like Govs. Ted Strickland of Ohio, Jennifer Granholm of Michigan and Ed Rendell of Pennsylvania.

Strickland described joint campaign events as "healing steps" toward uniting the two camps.

"I think for some people that were deeply involved in Senator Clinton's campaign, you know this is a time of some grieving perhaps," he said Thursday. "I have seen a coming together in this campaign that I think has happened earlier than I expected, and quite frankly, I think earlier than divisions that have occurred in primary campaigns in presidential elections past."

FOX News' Major Garrett and The Associated Press contributed to this report.