The Federal Bureau of Investigation said on Wednesday that a letter addressed to President Obama appeared to contain the poison ricin.

The letter was intercepted at a screening facility outside the White House, the Secret Service said.

It was received on Tuesday — similar timing to a letter addressed to Senator Roger Wicker, Republican of Mississippi, that tested positive for ricin. The letter addressed to the president had similar markings and was similar in appearance to the one addressed to Mr. Wicker, according to a law enforcement official.

In a statement, the F.B.I. said that the letter contained “a granular substance that preliminarily tested positive for ricin.” The bureau also said that ricin had been discovered at another mail screening facility.

The Secret Service did not disclose what was in the letter or provide any details, saying it was intercepted in a facility that “routinely identifies letters or parcels that require secondary screening or scientific testing before delivery.”

President Obama came here on Monday before a roaring, enthusiastic crowd to remember the tragedy of 20 children and 6 educators slain at Sandy Hook Elementary School and put new pressure on a recalcitrant Congress to honor them with gun-control legislation.

In an impassioned speech that at times took on the tone of a campaign rally, Mr. Obama told an audience of 3,100 at the University of Hartford that he came to Connecticut to ensure that the deaths in the school in Newtown would not recede and to remind Americans how important their voice is as the gun debates unfold.

“If you’re an American who wants to do something to prevent more families from knowing the immeasurable anguish that these families here have known, then we have to act,” Mr. Obama said. “Now’s the time to get engaged. Now’s the time to get involved. Now’s the time to push back on fear and frustration and misinformation. Now’s the time for everybody to make their voices heard, from every statehouse to the corridors of Congress.”

But as Mr. Obama spoke, Republicans on Capitol Hill were threatening to prevent a gun-control measure from even coming up for debate.

Senator Mitch McConnell of Kentucky, the Republican leader, announced Monday that he would join at least 13 other Republicans who have vowed to block consideration of gun legislation passed by the Senate Judiciary Committee and assembled by the Democratic leadership. That effectively made the threatened filibuster a test of Republican unity.

Mr. McConnell made his announcement as the Senate returned from recess and the legislative struggle over new gun safety legislation entered a critical phase. Senator Harry Reid of Nevada, the majority leader, took steps to force a vote to start a broad review of gun-control proposals and accused those threatening a filibuster of “blatant obstruction,” even as they showed no signs of backing down.

“Shame on them,” said Mr. Reid, a Democrat.

Repeatedly interrupted by applause, Mr. Obama asked his audience to recognize the Palestinian right to self-determination and justice that Israelis enjoy.

“Put yourself in their shoes - look at the world through their eyes,” he said. “It is not fair that a Palestinian child cannot grow up in a state of her own, and lives with the presence of a foreign army that controls the movements of her parents, every single day. It is not just when settler violence against Palestinians goes unpunished. It is not right to prevent Palestinians from farming their lands; to restrict a student’s ability to move around the West Bank; or to displace Palestinian families from their homes.

“Neither occupation nor expulsion is the answer,” he said. “Just as Israelis built a state in their homeland, Palestinians have a right to be a free people in their own land.”

He asked the audience to consider what kind of long-term future they want for their country, invoking the words of Ariel Sharon, the former Israeli military leader and prime minister who steered the country through multiple wars with Israel’s neighbors.

“It is impossible to have a Jewish, democratic state and at the same time to control all of Eretz Israel,” Mr. Obama said, quoting Mr. Sharon. “If we insist on fulfilling the dream in its entirety, we will lose it all.”

Hours after Palestinian militants fired at least two rockets from the Hamas-controlled Gaza Strip that crashed into the Israeli border city of Sderot on Thursday, President Obama traveled to the West Bank city of Ramallah and renewed his call for a two-state solution between Israelis and Palestinians, saying that continued Israeli settlement-building did not advance the cause of peace.

But, at a news conference with President Mahmoud Abbas, the leader of the Palestinian Authority and the secularist Fatah movement that rivals Hamas, Mr. Obama did not specifically call for a halt in settlement construction and urged both sides to press for a broad agreement that would meet two objectives: to provide sovereignty and a state for Palestinians and security for Israel.

“The core issue right now is how do we get sovereignty for the Palestinian people and security for Israeli people,” he said after almost two hours of talks with Mr. Abbas.

He added: “That’s not to say settlements aren’t important. That’s to say if we solve those two problems, the settlement issue will be resolved.”

Mr. Obama traveled to Ramallah after talks on Wednesday with Prime Minister Benjamin Netanyahu of Israel on the first day of his visit.

“I’ve been clear with Prime Minister Netanyahu and other Israeli leadership,” Mr. Obama said. “We do not consider continued settlement activity to be constructive, to be appropriate, to be something that can advance the cause of peace.”

Mr. Obama said Palestinians deserved an end to occupation and to the “daily indignities that come with it,” and a “future of hope.”

In short, he said, “Palestinians deserve a state of their own.”

The fact is the 12 hottest years on record have all come in the last 15. Heat waves, droughts, wildfires, floods — all are now more frequent and more intense. We can choose to believe that Superstorm Sandy and the most severe drought in decades and the worst wildfires some states have ever seen were all just a freak coincidence.

Or we can choose to believe in the overwhelming judgment of science — and act before it’s too late. …I urge this Congress to get together, (and) pursue a bipartisan, market-based solution to climate change, like the one John McCain and Joe Lieberman worked on a few years ago. But, if Congress won’t act soon to protect future generations, I will. I will direct my Cabinet to come up with executive actions we can take now and in the future to reduce pollution, prepare our communities for the consequences of climate change, and speed the transition to more sustainable sources of energy.

Franklin D. Roosevelt railed against the dangers posed by “coldblooded shooting” in 1934. Lyndon B. Johnson urged Congress to “stop the trade in mail-order murder” in 1968. Gerald Ford said he wanted to “make it harder to obtain cheap guns for criminal purposes” in 1976.

The problem of gun violence has been mentioned in State of the Union speeches on and off for decades, but tonight President Obama is expected to use his address to make the biggest call for new gun control laws since Bill Clinton, who made it a repeated theme of his speeches. (After the shooting of Representative Gabrielle Giffords in Tucson in 2011, Mr. Obama called for “working together,” not new gun laws.)

President Clinton called on Congress to pass the Brady bill, requiring background checks for many gun purchases, in his 1993 State of the Union address, urging lawmakers to “protect our families against the violent crime which terrorizes our people and which tears our communities apart.”

The following year Mr. Clinton called on Congress to ban assault weapons.
“I want to say something about this issue,’’ he said (LINK: ). “Hunters must always be free to hunt. Law-abiding adults should always be free to own guns and protect their homes. I respect that part of our culture; I grew up in it. But I want to ask the sportsmen and others who lawfully own guns to join us in this campaign to reduce gun violence. I say to you, I know you didn’t create this problem, but we need your help to solve it. There is no sporting purpose on earth that should stop the United States Congress from banishing assault weapons that out-gun police and cut down children.”

But some members of Congress paid a political price for those votes, as Mr. Clinton acknowledged in his 1995 speech, after Republicans won control of the House. “I don’t think it’s a secret to anybody in this room that several members of the last Congress who voted for that aren’t here tonight because they voted for it,’’ he said. “And I know, therefore, that some of you who are here because they voted for it are under enormous pressure to repeal it. I just have to tell you how I feel about it.”

“The members of Congress who voted for that bill and I would never do anything to infringe on the right to keep and bear arms to hunt and to engage in other appropriate sporting activities,’’ he said. “I’ve done it since I was a boy, and I’m going to keep right on doing it until I can’t do it anymore. But a lot of people laid down their seats in Congress so that police officers and kids wouldn’t have to lay down their lives under a hail of assault weapon attack, and I will not let that be repealed. I will not let it be repealed.”

In 1999 Mr. Clinton touted the bill’s success. “The Brady bill has stopped a quarter million felons, fugitives, and stalkers from buying handguns,’’ he said. “And now, the murder rate is the lowest in 30 years and the crime rate has dropped for six straight years.”