Barack Obama opted out of the federal public financing system Thursday, setting himself up for a huge cash advantage over John McCain in the general election -- but also providing his critics with a large opportunity to criticize his claim that he is committed to change in Washington.

Republicans slammed the presumptive Democratic presidential nominee's decision to abandon an earlier pledge to use the more than $84 million in public money if his Republican rival did the same.

McCain's campaign and supporters said the Illinois senator showed his true colors as a dyed-in-the-wool Washington player. And McCain said Thursday in Minnesota that "we will take public financing."

"Rather than sending a message that he's about change or a new type of politics in America, it really looks and sounds like he's about the same old ... type of politics in America," Republican Texas Sen. John Cornyn told FOX News.

"The thing that is most important in terms of an office holder or a politician's relationship with his constituents or the people is trust. And frankly when a politician says one thing and does another it erodes that trust," Cornyn said. "It's just another chink in the armor but a lot these things will add up over time."

The public financing system is paid for with $3 contributions that taxpayers can make in their returns -- but in accepting the money candidates are bound to an $84 million spending limit.

In Iowa, McCain criticized his rival for backtracking, reminding reporters that Obama "said he would stick to his word. He didn't." The Republican candidate added, "This election is about a lot of things. It's also about trust. It's about keeping your word."

A statement issued by the McCain campaign said Obama "has revealed himself to be just another typical politician who will do and say whatever is most expedient for Barack Obama ... Barack Obama is now the first presidential candidate since Watergate to run a campaign entirely on private funds."

Even Democratic Wisconsin Sen. Russ Feingold, who worked with Obama on an ethics and lobbying reform bill last year, called Obama's financing decision "a mistake" in a statement Thursday.

"This is not a good decision. While the current public financing system for the presidential primaries is broken, the system for the general election is not," he said.

Obama said in a November 2007 questionnaire by the Midwest Democracy Network that he would agree to forgo private funding for the November election if nominated. He elaborated by saying his plan would be to return excess donations and hold both major party candidates to a public "fundraising truce."

"Senator John McCain has already pledged to accept this fundraising pledge," Obama wrote. "If I am the Democratic nominee, I will aggressively pursue an agreement with the Republican nominee to preserve a publicly financed general election."

Having numerically clinched the GOP nomination, McCain says he will accept public funds for the general election -- but Obama has gradually eased off his earlier pledge.

Obama and his supporters argue his reason for doing so speaks to his ambitions as a reformer.

Obama has already "rewritten the book when it comes to financing campaigns" by rejecting Washington lobbyist and political action committee contributions, Illinois Sen. Dick Durbin, an Obama supporter, told FOX News.

Durbin said that with his broad-based network of small donors, "He's really behind reform and showing he can do it."

Obama, in his video announcement to supporters on Thursday, cast his approach as progressive, saying his supporters helped him build a "new kind of politics" where candidates don't rely on a small pool of mega-contributors from inside the Beltway.

Obama argued that the public financing system is "broken, and we face opponents who've become masters at gaming this broken system."

Obama said McCain and the Republican National Committee are fueled by contributions from Washington lobbyists and political action committees. He said McCain would also not stop the attacks from independent 527 groups.

But few Republican-leaning groups have weighed into the presidential contest so far. In fact, Obama allies are the ones who claim they are ready to shell out the big bucks to defeat McCain.

The AFL-CIO, which is expected to soon endorse Obama, has launched a campaign to link McCain with what officials say are Bush's failed economic policies. The group has $53 million at the ready to do so.

The Politico reported three months ago that liberal groups, including MoveOn.org, plan to spend up to $400 million to elect Democrats this fall.

Democracy 21, a non-profit group that focuses on the influence of money in politics, said in a statement it did not agree with Obama's "rationale" for bypassing the system. The group said it was "very disappointed" by the decision.

Republicans said Obama was just making excuses Thursday.

"Does the Obama campaign watch TV?" RNC spokesman Alex Conant said. "The independent ads currently airing are being sponsored by Obama's special interest allies at MoveOn.org attacking Senator McCain. Yet the Obama campaign has the audacity to falsely claim it has shutdown anti-McCain groups in order to justify abandoning its public financing pledge."

Currently, the RNC has far more cash on hand than the Democratic National Committee - and both parties can spend of behalf of their candidates. But Obama's personal advantage is daunting.

Having shattered campaign fundraising records, Obama raked in more than $265 million as of the end of April. Of that, nearly $10 million was for the general election. McCain on the other hand had raised nearly $115 million by the end of May. His campaign has returned all $3 million in general election contributions, urging donors to instead contribute to the RNC.

"Every day, Senator Obama morphs into a typical Washington politician," McCain adviser Frank Donatelli said. "I don't know what's more typical Washington than breaking a promise for short-term political advantage."

Obama timed the public financing decision with the announcement of his first general election television ad. The 60-second ad, to air in 18 states, is titled "Country I Love" and highlights how his single mother and grandparents taught him the "heartland" values of accountability and patriotism during his childhood.

Though McCain is set to take public financing for the fall election, the Democrat Party plans to file a new lawsuit to compel federal regulators to investigate whether McCain violated election laws by withdrawing from public financing during the primary.

At issue is a $4 million line of credit that the McCain campaign obtained late last year. While the loan was not secured by the promise of public funds, it required McCain to reapply for public funds if he lost early primary contests and to use that money as collateral.

In a complaint filed with the FEC in February, the national Democratic Party argued that the arrangement violated federal regulations — a claim McCain’s campaign and his bankers adamantly deny.

FOX News' Mike Majchrowitz and Mosheh Oinounou and The Associated Press contributed to this report.