Washington (CNN) -- Partisan tempers flared at a Senate immigration hearing on Monday as top Democrats accused opponents of comprehensive reform legislation of using last week's Boston Marathon bombings to slow or even derail the bill.
"Last week, opponents of comprehensive immigration reform began to exploit the Boston marathon bombing," said Senate Judiciary Committee Chairman Patrick Leahy, D-Vermont.
"I urge restraint in that regard. ... Let no one be so cruel as to try to use the heinous attacks of these two young men last week to derail the dreams and futures of millions of hard-working people," Leahy added.
He said the bill crafted by the bipartisan "Gang of Eight" would "serve to strengthen our national security by allowing us to focus our border security and enforcement efforts against those who would do us harm."
"A nation as strong as ours can welcome the oppressed and persecuted without making compromises in our security," he said. "We are capable of vigilance in our pursuit of these values, and we have seen the tremendous work that the local law enforcement as well as the federal law enforcement have done in the Boston area, and I am so proud of them."
Leahy's statement, delivered at the start of the hearing, was met with a sharp response from the panel's top Republican.
"I want you to take note of the fact that when you proposed gun legislation, I didn't accuse you of using the (Newtown, Connecticut,) killings as an excuse," said Sen. Chuck Grassley, R-Iowa.
"I don't hear any criticism ... when there (were) 14 people killed in West,Texas, and (some political activists took) advantage of that tragedy to warn about more government action to make sure that fertilizer factories are safe," Grassley said.
"I think we are taking advantage of an opportunity when, once in 25 years we deal with immigration, to make sure that every base is covered," he added.
Grassley became visibly agitated later in the hearing when Sen. Chuck Schumer, D-New York, said "those who are pointing to what happened -- the terrible tragedy in Boston -- (are looking for an) excuse for not doing a bill or delaying it many months or a year."
"I never said that!" Grassley yelled in response, interrupting Schumer.
"I didn't say you did, sir," Schumer replied. "I didn't mean you, Mr. Grassley."
Alabama GOP Sen. Jeff Sessions then accused Schumer of "demeaning" anti-reform witnesses at the hearing, partly by characterizing them as representatives of narrow interests.
Schumer, in turn, insisted that "what I am saying is, if there are things that come up as a result of what happened in Boston that need improving, that require improvement, let's add them to the bill because certainly our bill tightens up things in a way that would make a Boston less likely."
"That is all I am saying," Schumer concluded, "because I have heard lots of calls from people out in the country saying delay it."
Separately, Sen. Rand Paul, R-Kentucky, sent a letter Monday to Senate Majority Leader Harry Reid, D-Nevada, urging a delay of any immigration legislation until more is learned about the alleged Boston bombers.
"We should not proceed until we understand the specific failures of our immigration system," Paul wrote. "Why did the current system allow two individuals to immigrate to the United States from ... an area known as a hotbed of Islamic extremism, who then committed acts of terrorism?"
Asked about the letter, White House Press Secretary Jay Carney dismissed Paul's concern, arguing that "comprehensive immigration reform, as anyone who has looked into it can attest, would enhance our security."
Among other things, the bill now under consideration would prevent undocumented residents from gaining legal status if they have been found guilty of felonies or more than two misdemeanors.
Monday's hearing was the second by the Judiciary Committee on the nearly 900-page proposal put together by the eight senators. Over twenty witnesses were slated to testify, a schedule characterized by Sessions as part of a plan "to rush through this massive legislation before the American people know what's in it."
The first hearing, held last Friday, was also marked by several references to security fears raised by the recent bombings. Homeland Security Secretary Janet Napolitano is scheduled to testify before the panel Tuesday.
Assuming the immigration bill clears Leahy's committee, full Senate consideration of the legislation will probably occur in June, according to multiple sources.
A bipartisan group of legislators in the House of Representatives is crafting its own immigration reform proposal.