Senators pushing for an immigration overhaul are going on offense against the intensifying effort to leverage anxiety about the marathon bombing to derail momentum for the legislation.

“We’re not going to let them use what happened in Boston as an excuse,” Charles E. Schumer, D-N.Y., one of the “gang of eight” senators who wrote the bipartisan bill, said Sunday on CNN.

Several lawmakers and conservative advocates in recent days have suggested that it’s wrong to begin the immigration debate so soon after the Boston bombings. Without explicitly saying so, these critics have suggested that an overhaul they already view as too permissive would allow unsavory potential terrorists even easier entry into the United States.

But if that’s so, what’s known so far about the brothers who allegedly carried out the attack doesn’t seem to support such an argument. They arrived in the United States a decade ago, when Tamerlan Tsarnaev was the age of a sophomore in high school and Dzhokar Tsarnaev was a fourth-grader, almost certainly too young for them to have immigrated with actionable terrorist thoughts about their new home on their minds. The boys and two sisters emigrated from Russia, where they had arrived as refugees from Kyrgyzstan. They followed their parents, ethnic Chechens who had been granted political asylum in the United States.

Dzhokar, who’s now 19 and remains hospitalized and in serious condition after his capture on April 19, became an American citizen in September. Tamerlan, 26, who died in a shootout on April 19, was a legal resident.  How they were radicalized, the circumstances of Tamerlan’s travels in Russia for six months last year and the nature of the FBI’s earlier interviews with him at the Russian government’s request will be the subject of intense congressional and public interest in coming days. But, at least on the surface, their known path through the immigration system sounds more like an ushering through the proverbial golden door than a slipping through the cracks.

“Refugees and asylum seekers have enriched the fabric of this country from our founding,”  Chairman Patrick J. Leahy, D-Vt., said in convening a Senate Judiciary Committee hearing on the bill this morning. “Let no one be so cruel as to try to use the heinous acts of two young men last week to derail the dreams and futures of millions of hardworking people.”

Leahy and Schumer both used the same argument at the hearing that was used over the weekend by two “gang of eight” Republicans, Sens. John McCain of Arizona and Lindsey Graham of South Carolina. They all say the Boston bombing is a reason to accelerate the debate on legislation, not delay it, because of its exit visa requirements and other provisions designed to bolster immigration security.

More than anything else, they say, abandoning deportations and putting 11 million undocumented people on a pathway to citizenship would allow law enforcement to spend more time on border security and investigating immigrants’ terrorist threats.

The slow-walking-because-of-Boston call from the immigration overhaul’s critics may not get all that many adherents in the Senate. It was unveiled on April 19 by Judiciary’s top Republican, Charles E. Grassley, and espoused yesterday by Sen. Dan Coats, R-Ind., but so far there has not been any additional groundswell. The chances for that are likely to dim as the public comes to understand more of the facts about both the suspects and the bill.

But even a slight delay works to the advantage of opponents, who continue to assume a bill like the one from the “gang of eight” will win a solid majority in the Senate. They are already focusing almost as much lobbying attention on the more skittish, and more Republican, House.