Arms control proponents hailed the progress. Daryl G. Kimball, executive director of the Arms Control Association, called it “the first truly post-cold-war nuclear arms reduction treaty.” Richard Burt, a former chief Start negotiator who now heads a disarmament advocacy group called Global Zero, said that the two presidents “took a major step toward achieving their goal of global zero.”

The breakthrough ended nearly a year of tumultuous negotiations that dragged on far longer than anticipated. The two sides quarreled over verifying compliance, sharing telemetry and limiting missile defense programs. Mr. Obama restructured Mr. Bush’s plans for an antimissile shield in Europe, but Moscow objected to the new version as well and wanted restrictions. Mr. Obama refused. The two presidents cut through disagreements during a telephone call on March 13.

The treaty will go for ratification to the legislatures in both countries, and the politics of Senate ratification could be tricky, coming at a polarized moment with a midterm election on the horizon. Republican senators have already expressed concern that Mr. Obama might make unacceptable concessions. Ratification in the Senate requires 67 votes, meaning Mr. Obama would need support from Republicans.

Senators Mitch McConnell of Kentucky and Jon Kyl of Arizona, the Republican leaders, wrote Mr. Obama last week warning him that ratification “is highly unlikely” if the treaty contained any binding linkage between offensive weapons and missile defense, reminding him of his position “that missile defense is simply not on the table.”

Administration officials describing the draft treaty said its preamble recognized the relationship between offensive weapons and missile defense, but that the language was not binding. The treaty establishes a new regime of inspections, but the American monitoring team that was based at the Votkinsk missile production factory until Start expired would not be allowed to return on a permanent basis.

Russian analysts said Moscow was happy to have reduced what it saw as the overly intrusive inspection regime mandated by Start but disappointed not to have secured restrictions on missile defense. The military was pressuring the Kremlin not to agree to arms reductions without limits on the American missile shield, even though both Mr. Bush and Mr. Obama have described it as aimed at Iran, not Russia. In the end, the Kremlin overruled the military because it wanted a foreign policy achievement. “The military does not have the influence that it did during Soviet times,” said Anton V. Khlopkov, director of the Center for Energy and Security Studies in Moscow. “Back then, the military people, if they didn’t run, they were among those who led the arms control negotiations from the Soviet side. Now, they have less of a role.”

Vladimir Z. Dvorkin, a retired major general and arms control adviser, said Moscow would retain the ability to scrap the new treaty if American missile defenses became a threat. “If, for example, the U.S. unilaterally deploys considerable amounts of missile defense, then Russia has the right to withdraw from the agreement because the spirit of the preamble has been violated,” he said.

Mr. Obama met at the White House on Wednesday with Senators John Kerry of Massachusetts and Richard G. Lugar of Indiana, the senior Democrat and Republican on the Senate Foreign Relations Committee, to brief them on the negotiations. Mr. Kerry later said he would hold hearings between Easter and Memorial Day on the history of arms control and promised action by year’s end. “I assured the president that we strongly support his efforts and that if the final negotiations and all that follows go smoothly, we will work to ensure that the Senate can act on the treaty this year,” Mr. Kerry said.