WASHINGTON — “Time is running out” for the tactical gains from President Bush’s surge strategy to produce lasting political progress in Iraq, Sen. Jack Reed said from Baghdad yesterday, as one of this year’s deadliest insurgent bombings hit north of the capital city.

On a three-day tour of the region, Reed said his talks with top U.S. officials and his meetings with reconstruction and fighting units have deepened his belief that U.S. troop withdrawals should begin this fall, as a starting point for a shift to a markedly smaller and more modest mission in the war by spring.

The Rhode Island Democrat, who spoke in a conference call with reporters, said U.S. commanders and diplomats were aware of the waning public support for the war as the Senate prepares to renew debate about troop withdrawals this week.

Reed said yesterday’s suicide truck bombing, which killed more than 105 people in a farming town about 100 miles north of Baghdad, was fresh proof that insurgents “still have the capacity to mount — not as often, but often enough — these spectacular, high-profile attacks.”

Reed said Mr. Bush’s surge strategy has produced some limited “tactical success” in reducing the insurgent attacks in certain areas, particularly around Baghdad.

Reed said he visited a fighting unit outside Baghdad that has been “very effective at stopping the transit of insurgents” in and out of the capital city.

In the city of al-Hillal, Reed said he met with a provincial reconstruction team — one of the groups of U.S. civilians charged with helping to rebuild Iraq’s public works and to strengthen its local political institutions. “Their effort is under way,” Reed said, but “they still have not turned the corner,” especially in areas of poor security from violence.

Reed also pointed to significant military progress in the largely Sunni province of al-Anbar in the west of Iraq, where tribal leaders have turned against al-Qaida and begun to cooperate closely with U.S. and Iraqi government forces.

But Reed said these hopeful signs from this year’s surge of 28,000 U.S. troops — putting the total near 160,000 — cannot offset two countervailing forces: the Iraqi government’s failure to move toward a reconciliation of the nation’s warring forces, and the limited U.S. ability to sustain its force in Iraq.

Where the surge has succeeded, Reed said insurgents have chosen to pull back and bide their time. Meantime — as in yesterday’s truck bombing — they stage attacks beyond the reach of the augmented U.S. force, he said.

Reed said there remained a “real question” whether the Shiite government will ever accept the Sunnis, and whether such a government will ever be able to deliver the necessary services to its people.

In the large expanses of Iraq where Sunnis are in the minority, “there is great reluctance” on the part of the Shiite-led government to accept the Sunnis as national partners.

For example, Reed noted that the Iraqi government was still struggling to enact a broadly acceptable plan for the sharing of the nation’s oil wealth among its contesting ethnic and religious groups.

Reed expects to learn more on that score today, when he travels to Irbil, the capital of Kurdish Iraq. This semiautonomous region in the north of Iraq is home to large hydrocarbon reserves, which the Kurds, long oppressed under Saddam Hussein’s regime, are intent on exploiting.

After meeting and dining yesterday with Gen. David Petraeus, the commander of U.S. forces in Iraq, and U.S. Ambassador Ryan Crocker, Reed said U.S. leaders in Iraq were “quite aware” of the weakening support for the war in Congress, and of what he called the “palpable public sentiment that change is in order.”

Reed said Petraeus and his associates were considering a wide range of options for an advisory report that they intend to offer President Bush and the Congress next month on the path ahead in Iraq. “It is quite clear to me that they have not yet formulated their advice,” Reed said.

Reed said he thought it was “realistic to begin to bring our troops home within 120 days” because “we’re got to establish the policy that we are beginning reductions.” He will offer legislation to that effect this week.

If the United States scales down its force and mission, Reed was asked, how can the insurgents be prevented from rushing back and disrupting areas the Americans have secured?

“I would hope that the Iraqi security forces could be put into the mix and that they could help,” Reed answered. He also said that the U.S. forces could also try to lend air power against insurgents.

When Congress returns this week from its Fourth of July recess, Reed and other key Senate Democrats will try to use the debate over next year’s defense authorization bill to legislate a change in course in Iraq.

With Sen. Carl Levin, the chairman of the Senate Armed Services Committee, Reed will again offer an amendment requiring troop withdrawals from Iraq to begin, with a nonbinding goal of shifting to a more modest mission — and a far smaller force — by next spring.

Such an amendment won the support of a majority during the spring debate over an urgent bill to continue paying the cost of the wars in Iraq and Afghanistan. But Democrats could not win enough votes to override President Bush’s veto of an overall war-spending bill that contained mandatory troop-withdrawal deadlines.

But Congress will reconvene in “a different environment than even a month or so ago,” Reed said. Members of Congress have been home to hear from the public, Reed said. He added that he perceived “a growing sense within the administration, even” that the current war effort in Iraq “will be difficult to sustain indefinitely.”

jmulligan@belo-dc.com