WASHINGTON — Fresh off his protracted fight over diplomacy with Senator Clinton, Senator Obama will seek to bolster his national security credentials today with a major Washington speech on counterterrorism.

The address is titled “The War We Need To Win,” and in it Mr. Obama will discuss how the war in Iraq and missteps by the Bush administration have made America “less safe” since the attacks of September 11, 2001, as well as his plans to “fight the right war on the right battlefield,” according to the Illinois senator’s campaign. The speech, which his campaign insists has been long scheduled, signals both Mr. Obama’s willingness to escalate the foreign policy debate and his need to avoid appearing weak on terrorism in light of his commitment to meet with foreign dictators.

While Mrs. Clinton refused a pledge at last Monday’s Democratic debate to meet with rogue leaders such as Fidel Castro and Hugo Chavez, Mr. Obama said he would, prompting the first direct policy battle of the campaign. The candidates traded shots for nearly a week, with the former first lady calling Mr. Obama “irresponsible” and “naïve,” and Mr. Obama comparing Mrs. Clinton’s diplomatic approach to “Bush-Cheney lite.”

Mr. Obama has embraced the debate as a validation of the “fresh” style he has vowed to bring to Washington, but the Clinton campaign has also used it to reinforce the perception that she is more experienced and prepared to be commander in chief. “He’s got to prove he’s strong and tough,” particularly when it comes to the war on terrorism, a Democratic political consultant, Hank Sheinkopf, said. If not, he said, Mr. Obama’s foreign policy approach risks comparisons to a president like Jimmy Carter, along with a fight Republicans would be eager to wage.

Mr. Sheinkopf said Mr. Obama has won support thus far largely from a younger demographic, which is less likely to vote in high numbers, meaning that if he wants to win, he will have to appeal to the older voters that may lean to Mrs. Clinton. “He’s got to talk to an adult audience,” Mr. Sheinkopf said.

The counterterrorism speech comes a day after the Obama campaign released an ad to run in Iowa aimed at drawing attention to Mr. Obama’s support for ethics reform. The 30-second spot shows him taking on lobbyists, and a narrator notes that he refuses to accept their campaign contributions or those from political action committees.

“They think they own this government,” Mr. Obama says in a clip shown in the ad from his February announcement speech. “But we’re here today to take it back.”

The spot casts the senator as a Washington outsider and it directly confronts questions about his experience.

“I know that I haven’t spent a lot of time learning the ways of Washington,”

Mr. Obama says in a clip that he repeats often on the stump. “But I’ve been there long enough to know that the ways of Washington must change.”